Separation of duty is necessarily determined by conditions external to the computer system. The Clark-Wilson scheme includes the requirement that the system maintain the separation of duty requirement expressed in the access control triples. Enforcement is on a per-user basis, using the user ID from the access control triple.

A MAC is used to authenticate files between users. If the sender
and receiver both have the secret key, they are the only ones that can
verify the hash function. If a symmetric key algorithm is used to
encrypt the one-way hash function, then the one-way hash function
becomes a keyed MAC.
Answer "Generating a digital signature" is incorrect because a digital signature between two parties uses an asymmetric key algorithm. If a message is encrypted with the sender's private key, then only the sender's public key can decrypt the message. This proves that the message was sent by the sender since only the sender knows the private key.
In practice, asymmetric key encryption is very slow, especially for
long messages. Therefore, a one-way hash of the message is
encrypted with the sender's private key instead of encrypting the
complete message. Then, the message and the encrypted hash are
sent to a second party. The receiver takes the encrypted hash and
decrypts it with the sender's public key. Then, the receiver takes the
hash of the message, using the same one-way hash algorithm as
the sender. The hash generated by the receiver is compared with the
decrypted hash sent with the message. If the two hashes are identical,
the digital signature is validated. Note that his method also will
reveal if the message was changed en route, since the hash calculated
by the receiver will, then, be different from the encrypted hash sent
along with the message.
Answer "Providing for secrecy of the message" is incorrect since encrypting the hash of the message and sending the message in the clear does nothing to protect the confidentiality of the message. Since the hash function is a one-way function, the message cannot be recovered from its hash.
Answer "Generating a one-way function" is incorrect since encrypting a hash of a message is not a one-way function. If it were, it would be of no use since no one would be able to reverse the process and decrypt it.

The best location in a network to place Virtual Private Network (VPN) devices when an internal review reveals network design flaws in remote access is in a dedicated Demilitarized Zone (DMZ). A DMZ is a network segment that is located between the internal network and the external network, such as the internet. A DMZ is used to host the services or devices that need to be accessed by both the internal and external users, such as web servers, email servers, or VPN devices. A VPN device is a device that enables the establishment of a VPN, which is a secure and encrypted connection between two networks or endpoints over a public network, such as the internet. Placing the VPN devices in a dedicated DMZ can help to improve the security and performance of the remote access, as well as to isolate the VPN devices from the internal network and the external network. Placing the VPN devices in its own separate VLAN, at the ISP, or outside the external firewall are not the best locations, as they may expose the VPN devices to more risks, reduce the control over the VPN devices, or create a single point of failure for the remote access. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 6: Communication and Network Security, page 729; Official (ISC)2 Guide to the CISSP CBK, Fifth Edition, Chapter 4: Communication and Network Security, page 509.

Effective user involvement is the most critical factor in ensuring that the application
meets business objectives.
A great way of getting early input from the user community is by using Prototyping. The
prototyping method was formally introduced in the early 1980s to combat the perceived
weaknesses of the waterfall model with regard to the speed of development. The objective is to
build a simplified version (prototype) of the application, release it for review, and use the feedback
from the users' review to build a second, better version.
This is repeated until the users are satisfied with the product. t is a four-step process:
initial concept,
design and implement initial prototype,
refine prototype until acceptable, and
complete and release final version.
There is also the Modified Prototype Model (MPM. This is a form of prototyping that is ideal for
Web application development. It allows for the basic functionality of a desired system or
component to be formally deployed in a quick time frame. The maintenance phase is set to begin
after the deployment. The goal is to have the process be flexible enough so the application is not
based on the state of the organization at any given time. As the organization grows and the
environment changes, the application evolves with it, rather than being frozen in time.
Reference(s) used for this question:
Hernandez CISSP, Steven (2012-12-21). Official (ISC)2 Guide to the CISSP CBK, Third Edition
((ISC)2 Press) (Kindle Locations 12101-12108 and 12099-12101). Auerbach Publications. Kindle
Edition.
and
Information Systems Audit and Control Association, Certified Information Systems Auditor 2002
review manual, chapter 6: Business Application System Development, Acquisition, Implementation
and Maintenance (page 296).