Final answer:
Rats have the capability to compress their bodies and squeeze through spaces as small as the diameter of a tennis ball.
Step-by-step explanation:
The question pertains to the physical capabilities of rats and how small of an opening they can compress their bodies to squeeze through. After researching the flexible skeletal structure of rats, it is established that they can compress their bodies enough to fit through spaces as small as the diameter of a tennis ball. Therefore, the correct answer from the provided options is 1) A tennis ball.