The DMZ is a buffer between the protected and unprotected network.
"A part of a network perfectly safe from hackers" is incorrect. There is no such thing.
"A militarized network segment" is incorrect. While the term DMZ originated in the Korean
War, it has nothing to do with the military.
"A firewall" is incorrect. Firewalls can play an important part in building a DMZ but a DMZ is much more than a firewall.
References:
CBK, p. 850
AIO, p. 483

This the best of the four answers as a defense that depends on multiple layers is superior to one where all protection is embedded in a single layer (e.g., a firewall).
Defense in depth would include all categories of controls.
The Following answers are incorrect:
"Concept of a pass through device that only allows certain traffic in and out" is incorrect.
This is one definition of a firewall which can be a component of a defense in depth strategy in combination with other measures.
"Concept of preventative controls" is incorrect. This is a component of a defense in depth strategy but the core concept is that there must be multiple layers of defenses.
"Concept of defensive controls" is incorrect. This is a component of a defense in depth strategy but the core concept is that there must be multiple layers of defenses.
References:
http://en.wikipedia.org/wiki/Defense_in_depth_(computing)
http://www.nsa.gov/snac/support/defenseindepth.pdf

The correct answer is "The testing or reconciliation of evidence of a user's identity". Answer
"The means by which a user provides a claim of his or her identity to a system" is identification, "A system's capability to determine the actions and behavior of a single individual within a system" is accountability,
and "The rights and permissions granted to an individual to access a computer resource" is authorization.