It must be noted that Halon is now banned in most countries or cities.
The reason CO2 is preferred in an information center is the agent is considered a clean agent, as well as non-conductive. The agent evaporates and does not leave a residue on the equipment. CO2 can be hazardous to people so special care must be taken when implemented.
Water may be a sound solution for large physical areas such as warehouses, but it is entirely inappropriate for computer equipment. A water spray can irreparably damage hardware more quickly than encroaching smoke or heat. Gas suppression systems operate to starve the fire of oxygen. In the past, Halon was the choice for gas suppression systems; however, Halon leaves residue, depletes the ozone layer, and can injure nearby personnel.
NOTE FROM CLEMENT:
For the purpose of the exam do not go outside of the 4 choices presented. YES, it is true that there are many other choices that would be more adequate for a Data Centre. An agent such as IG-55 from Ardent would probably be a better choice than CO2, however it is
NOT in the list of choices.
You will also notice that Shon Harris and Krutz and Vines disagree on which one is the best. This is why you must do your own research to supplement the books, sometimes books could be opiniated as well. When in doubt refer to the official book and look at what is ISC2 view of the topic and which one ISC2 considers to be the best for the exam.
ISC2 recommends also the following:
Aero-K - uses an aerosol of microscopic potassium compounds in a carrier gas released from small canisters mounted on walls near the ceiling. The Aero-K generators are not pressurized until fire is detected. The Aero-K system uses multiple fire detectors and will not release until a fire is "confirmed" by two or more detectors (limiting accidental discharge). The gas is non-corrosive, so it does not damage metals or other materials. It does not harm electronic devices or media such as tape or discs. More important, Aero-K is nontoxic and does not injure personnel.
FM-200 - is a colorless, liquefied compressed gas. It is stored as a liquid and dispensed into the hazard as a colorless, electrically non-conductive vapor that is clear and does not obscure vision. It leaves no residue and has acceptable toxicity for use in occupied spaces at design concentration. FM-200 does not displace oxygen and, therefore, is safe for use in occupied spaces without fear of oxygen deprivation.
The following are incorrect choices:
Water or Soda/Acid & Halon: (old water extinguishers) will damage sensitive equipment as well as conduct electricity which could endanger the life of the person using such a fire extinghisher. Halon has been banned due to the Montreal Protocol.
ABC rated Dry chemical extinguishers: They are suitable for electrically energized fires, but they are not acceptable on sensitive equipment. It is like throwing a couple kilograms of flour in around in a room. It is extremely hard to clean off of equipment and some of the chemicals are corrosive in nature.
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 25609-25612). Auerbach Publications. Kindle
Edition.
and
http://www.ehs.ucf.edu/labsafe/safemgequip.html
or
http://www.osha.gov/doc/outreachtraining/htmlfiles/extmark.html

Explanation/Reference:
Explanation:
A covert channel is a way for an entity to receive information in an unauthorized manner. It is an information flow that is not controlled by a security mechanism.
Covert channels are of two types: storage and timing.
A covert storage channel involves direct or indirect reading of a storage location by another process. A covert timing channel depends upon being able to influence the rate that some other process is able to acquire resources, such as the CPU.
A covert storage channel is a "covert channel that involves the direct or indirect writing of a storage location by one process and the direct or indirect reading of the storage location by another process.
Covert storage channels typically involve a finite resource (e.g. sectors on a disk) that is shared by two subjects at different security levels.
A covert timing channel is a "covert channel in which one process signals information to another by modulating its own use of system resources (e.g. CPU time) in such a way that this manipulation affects the real response time observed by the second process
Incorrect Answers:
B: Software and Timing are not defined types of covert channels.
C: Kernel is not a defined type of covert channel.
D: Kernel is not a defined type of covert channel.
References:
http://www.isg.rhul.ac.uk/~prai175/ISGStudentSem07/CovertChannels.ppt
Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, pp. 378-379