Final answer:
Adult mice can fit through a 1 inch sized opening.
Step-by-step explanation:
Adult mice are agile and can fit through small openings to gain access to a structure. The correct answer would be A. 1 inch, as this is the size of the opening that adult mice can easily squeeze through. Openings that are smaller than 1 inch, such as 1/2 inch, 3/4 inch, and 1/4 inch, would be too small for adult mice to fit through.