What is the minimum number of disk required for disk mirroring?

A.
2

B.
3

C.
4

D.
5

Explanation:
Mirroring requires a minimum of two disks. Striping and parity across all disks requires a minimum of three disks.
Incorrect
Answers:
A: A minimum of three disks is required for striping with parity across all disks.
Mirroring only requires two disks.
C, D: It is possible to use any multiple of two disks for mirroring, but the minimum is two disks.
References:

Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 184-192.
David Groth and Toby Skandier, Network+ Study Guide (4th Edition), Sybex, Alameda CA, 2005, pp. 381-382.