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.
Chosen Answer:
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