8. Which one of the following words is not an object of a preposition?

a. pockets

b. bed

c. cushions

d. afternoon

Preposition is a word that indicates location so with that said ...

pocket: I put the keys in my pocket (location)

bed: I put my shirt on my bed (location)

cushions: I set the vase down on the cushion (location?)

afternoon is a time. so the answer is afternoon