Controls like guards and general steps to maintain building security, securing of
server rooms or laptops, the protection of cables, and usage of magnetic switches on doors and
windows are all examples of Physical Security.
Reference(s) used for this question:
KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of
Computer Security, 2001, John Wiley & Sons, Page 33.

Explanation/Reference:
Explanation:
This question is asking what the opposite of plain text is. In the context of information security, plain text means unencrypted text. The opposite of plain text is cipher text. Cipher text is another term for encrypted text.
Encryption is a method of transforming readable data, called plaintext, into a form that appears to be random and unreadable, which is called ciphertext. Plaintext is in a form that can be understood either by a person (a document) or by a computer (executable code). Once it is transformed into ciphertext, neither human nor machine can properly process it until it is decrypted. This enables the transmission of confidential information over insecure channels without unauthorized disclosure.
Incorrect Answers:
B: This answer is close but incorrect. Plaintext is readable data. The opposite of that is encrypted data (known as ciphertext), not 'encryption'.
C: Unplain text is not a valid term.
D: Digitally Signed is not the opposite of plaintext.
References:
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 765

A Certificate Revocation List (CRL) is periodically update by a Certificate Authortity (CA), where there may be a delay from the time a certificate has been revoked versus the time it
propgates into the Certificate Revocation List (CRL).
The CA can revoke certificates and provide an update service to the other members of the PKI via
a certificate revocation list (CRL), which is a list of non-valid certificates that should not be
accepted by any member of the PKI. The use of public key (asymmetric) cryptography has
enabled more effective use of symmetric cryptography as well as several other important features,
such as greater access control, nonrepudiation, and digital signatures.
In transactions where there is a need for real-time checks, the Online Certificate Status Protocol
can be used which can obtain the revocation status in a more timely fashion.
From RFC 2560
In lieu of or as a supplement to checking against a periodic CRL, it may be necessary to obtain
timely information regarding the revocation status of a certificate (cf. [RFC2459], Section 3.3).
Examples include high-value funds transfer or large stock trades.
The Online Certificate Status Protocol (OCSP) enables applications to determine the (revocation)
state of an identified certificate. OCSP may be used to satisfy some of the operational
requirements of providing more timely revocation information than is possible with CRLs and may
also be used to obtain additional status information. An OCSP client issues a status request to an
OCSP responder and suspends acceptance of the certificate in question until the responder
provides a response.
This protocol specifies the data that needs to be exchanged between an application checking the
status of a certificate and the server providing that status.
The following answers are incorrect:
-
The OCSP (Online Certificate Status Protocol) is a propietary certifcate mechanism developed by Microsoft and a Certificate Revocation List (CRL) is an open standard.
-
The OCSP (Online Certificate Status Protocol) is used only by Active Directory and a Certificate Revocation List (CRL) is used by Certificate Authorites.
-
The OCSP (Online Certificate Status Protocol) is a way to check the attributes of a certificate and a Certificate Revocation List (CRL) is used by Certificate Authorites.
The following reference(s) were/was used to create this question:
Schneiter, Andrew (2013-04-15). Official (ISC)2 Guide to the CISSP CBK, Third Edition :
Cryptography (Kindle Locations 2256-2259). . Kindle Edition.
http://www.ietf.org/rfc/rfc2560.txt
http://technet.microsoft.com/en-us/library/cc731027(v=ws.10)
http://www.networkworld.com/reviews/2004/0809revside.html