A rat can compress its body and squeeze through an opening as small as:

Rats are remarkably flexible creatures, capable of compressing their bodies to fit through surprisingly small openings. The correct choice indicates that a rat can squeeze through an opening as small as 1/2 inch. This adaptability is due to their anatomical structure; rats do not have a rigid skeletal structure like humans do; instead, their ribs are flexible, allowing them to flatten their bodies.

This ability to fit through tight spaces is a survival mechanism, enabling them to evade predators and access food sources. The dimensions highlighted in the other options are either larger than what is necessary or outside the rat's capabilities. Recognizing the correct dimension can assist in pest control efforts and understanding how to rat-proof structures effectively.