Final answer:
Adult rats can enter a structure through an opening that is at least 1 inch in size.
Step-by-step explanation:
The correct answer is A) 1 inch. Adult rats can enter a structure through an opening that is at least 1 inch in size. Rats are small and agile creatures that can squeeze through tight spaces and openings, so it is important to ensure that any entry points in a structure are sealed to prevent rat infestations.