Which detail from the text best supports the inference that one bad apple can spoil all the others?

. . . it is vitally important to store hard fruit through the whole year.
Place them carefully in your fruit house or “hoard house” on clean dry straw . . .
. . . make sure they are not touching each other . . .
. . . turn them very carefully every month to avoid their collecting moisture

The detail from the text that best supports the inference that one bad apple can spoil all the others is . . . make sure they are not touching each other . . .

What is an Inference?

This refers to the conclusion that can be gotten about a thing based on available evidence.

Hence, we can see that The detail from the text that best supports the inference that one bad apple can spoil all the others is . . . make sure they are not touching each other . . .