S/MIME (for Secure MIME, or Secure Multipurpose Mail Extension) is a security process used for e-mail exchanges that makes it possible to guarantee the confidentiality and non-repudiation of electronic messages.
S/MIME is based on the MIME standard, the goal of which is to let users attach files other than ASCII text files to electronic messages. The MIME standard therefore makes it possible to attach all types of files to e-mails.
S/MIME was originally developed by the company RSA Data Security. Ratified in July 1999 by the IETF, S/MIME has become a standard, whose specifications are contained in RFCs
2630 to 2633.
How S/MIME works
The S/MIME standard is based on the principle of public-key encryption. S/MIME therefore makes it possible to encrypt the content of messages but does not encrypt the communication.
The various sections of an electronic message, encoded according to the MIME standard, are each encrypted using a session key.
The session key is inserted in each section's header, and is encrypted using the recipient's public key. Only the recipient can open the message's body, using his private key, which guarantees the confidentiality and integrity of the received message.
In addition, the message's signature is encrypted with the sender's private key. Anyone intercepting the communication can read the content of the message's signature, but this ensures the recipient of the sender's identity, since only the sender is capable of encrypting a message (with his private key) that can be decrypted with his public key.
Reference(s) used for this question:
http://en.kioskea.net/contents/139-cryptography-s-mime
RFC 2630: Cryptographic Message Syntax;
OPPLIGER, Rolf, Secure Messaging with PGP and S/MIME, 2000, Artech House;
HARRIS, Shon, All-In-One CISSP Certification Exam Guide, 2001, McGraw-Hill/Osborne, page 570;
SMITH, Richard E., Internet Cryptography, 1997, Addison-Wesley Pub Co.

A security domain is a domain of trust that shares a single security policy and single management.
The term security domain just builds upon the definition of domain by adding the fact that resources within this logical structure (domain) are working under the same security policy and managed by the same group.
So, a network administrator may put all of the accounting personnel, computers, and network resources in Domain 1 and all of the management personnel, computers, and network resources in Domain 2. These items fall into these individual containers because they not only carry out similar types of business functions, but also, and more importantly, have the same type of trust level. It is this common trust level that allows entities to be managed by one single security policy.
The different domains are separated by logical boundaries, such as firewalls with ACLs, directory services making access decisions, and objects that have their own ACLs indicating which individuals and groups can carry out operations on them.
All of these security mechanisms are examples of components that enforce the security policy for each domain. Domains can be architected in a hierarchical manner that dictates the relationship between the different domains and the ways in which subjects within the different domains can communicate. Subjects can access resources in domains of equal or lower trust levels.
The following are incorrect answers:
The reference monitor is an abstract machine which must mediate all access to subjects to objects, be protected from modification, be verifiable as correct, and is always invoked.
Concept that defines a set of design requirements of a reference validation mechanism
(security kernel), which enforces an access control policy over subjects' (processes, users) ability to perform operations (read, write, execute) on objects (files, resources) on a system. The reference monitor components must be small enough to test properly and be tamperproof.
The security kernel is the hardware, firmware and software elements of a trusted computing base that implement the reference monitor concept.
The security perimeter includes the security kernel as well as other security-related system functions that are within the boundary of the trusted computing base. System elements that are outside of the security perimeter need not be trusted. not every process and resource falls within the TCB, so some of these components fall outside of an imaginary boundary referred to as the security perimeter. A security perimeter is a boundary that divides the trusted from the untrusted. For the system to stay in a secure and trusted state, precise communication standards must be developed to ensure that when a component within the
TCB needs to communicate with a component outside the TCB, the communication cannot expose the system to unexpected security compromises. This type of communication is handled and controlled through interfaces.
Reference(s) used for this question:
Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition (Kindle Locations
2 8548-28550). McGraw-Hill. Kindle Edition.
Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition (Kindle Locations
7 873-7877). McGraw-Hill. Kindle Edition.
Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition , Access Control,
Page 214-217
Schneiter, Andrew (2013-04-15). Official (ISC)2 Guide to the CISSP CBK, Third Edition :
Security Architecture and Design (Kindle Locations 1280-1283). . Kindle Edition.
TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation.
AIO 6th edition chapter 3 access control page 214-217 defines Security domains.
Reference monitor, Security Kernel, and Security Parameter are defined in Chapter 4,
Security Architecture and Design.

Trigger-based review and certification is the best way to prevent access privilege creep. Access privilege creep is the accumulation of unnecessary or excessive permissions by a user or an account over time, due to changes in roles, responsibilities, or assignments. Access privilege creep can pose a security risk, as it may violate the principle of least privilege and increase the attack surface or the potential for misuse. Trigger-based review and certification is a process of reviewing and validating the access privileges of a user or an account whenever a triggering event occurs, such as a role change, a project completion, a transfer, or a termination. Trigger- based review and certification can help prevent access privilege creep by ensuring that the access privileges are updated and aligned with the current needs and duties of the user or the account.