If you can't restore lost files from your backup system then your backup plan is useless. You could have the best backup system and plan available but if you are unable to restore files then the system can't assure data availability.
Develop an effective disaster recovery plan and include in that plan a good backup strategy that meets the needs of your organization. Be sure to include periodic recovery practice operations to prove the effectiveness of the system.
The following answers are incorrect:
-An effective backup plan: This is vital but testing the plan with restores is vital to operate a network safely.
-A reliable network infrastructure: This is incorrect because it is only part of what you need to have an effective backup and restore plan.
-Expensive backup hardware: This is good to have but if you don't rest your restore plan and it doesn't work when you need it, it is useless.
The following reference(s) was used to create this question: 2013. Official Security+ Curriculum.

A one way hashing function can only be use for the integrity of a message and not for authentication or confidentiality. Because the hash creates just a fingerprint of the message which cannot be reversed and it is also very difficult to create a second message with the same hash.
A hash by itself does not provide Authentication. It only provides a weak form or integrity. It would be possible for an attacker to perform a Man-In-The-Middle attack where both the hash and the digest could be changed without the receiver knowing it.
A hash combined with your session key will produce a Message Authentication Code (MAC) which will provide you with both authentication of the source and integrity. It is sometimes referred to as a Keyed Hash. A hash encrypted with the sender private key produce a Digital Signature which provide authentication, but not the hash by itself.
Hashing functions by themselves such as MD5, SHA1, SHA2, SHA-3 does not provide
authentication.
Source: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2001,
Page 548

A Polymorphic virus produces varied but operational copies of itself in hopes of
evading anti-virus software.
The following answers are incorrect:
boot sector. Is incorrect because it is not the best answer. A boot sector virus attacks the boot
sector of a drive. It describes the type of attack of the virus and not the characteristics of its
composition.
parasitic. Is incorrect because it is not the best answer. A parasitic virus attaches itself to other
files but does not change its characteristics.
stealth. Is incorrect because it is not the best answer. A stealth virus attempts to hide changes of
the affected files but not itself.