The correct answer is reducing the amount of data to be evaluated
by definition. Answer "Limit the number of letters in a password" is incorrect because clipping levels do not relate to letters in a password.
Answer "Set thresholds for voltage variations" is incorrect because clipping levels in this context have nothing to do with controlling voltage levels. Answer "Limit errors in callback syste" is incorrect because they are not used to limit callback errors.

The correct answer to this "security". It is a major factor in deciding if a centralized or decentralized environment is more appropriate.
Example: In a centralized computing environment, you have a central server and workstations (often "dumb terminals") access applications, data, and everything else from that central servers. Therefore, the vast majority of your security resides on a centrally managed server. In a decentralized (or distributed) environment, you have a collection of
PC's each with their own operating systems to maintain, their own software to maintain, local data storage requiring protection and backup. You may also have PDA's and "smart phones", data watches, USB devices of all types able to store data... the list gets longer all the time.
It is entirely possible to reach a reasonable and acceptable level of security in a distributed environment. But doing so is significantly more difficult, requiring more effort, more money, and more time.
The other answers are not correct because:
scalability - A distributed computing environment is almost infinitely scalable. Much more so than a centralized environment. This is therefore a bad answer.
heterogeneity - Having products and systems from multiple vendors in a distributed environment is significantly easier than in a centralized environment. This would not be a
"challenge of distributed computing solutions" and so is not a good answer.
usability - This is potentially a challenge in either environment, but whether or not this is a problem has very little to do with whether it is a centralized or distributed environment.
Therefore, this would not be a good answer.
Reference:
Official ISC2 Guide page: 313-314
All in One Third Edition page: (unavailable at this time)

The correct answer is "Private addresses can easily be routed globally" Private addresses are not easily routable; hence the reason for using NAT.

Explanation/Reference:
White-box testing is a method of testing software that tests internal structures or workings of an application, versus its functionality. White-box testing allows access to program source code, data structures, variables, etc.
Incorrect Answers:
B: Parallel Testing is the process of entering the same inputs in two different versions of the application and reporting the anomalies.
C: Regression Testing is the process of rerunning a portion of a test scenario or test plan to ensure that changes or corrections have not introduced new errors.
D: Pilot Testing is a preliminary test that focuses on specific and predefined aspect of a system.
References:
Conrad, Eric, Seth Misenar, Joshua Feldman, CISSP Study Guide, 2nd Edition, Syngress, Waltham, 2012, p. 194 Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 1105
https://en.wikipedia.org/wiki/White-box_testing
http://www.tutorialspoint.com/software_testing_dictionary/parallel_testing.htm
http://soft-engineering.blogspot.co.za/2010/12/what-is-difference-between-pilot-and.html