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