Our teacher told us a story about a pony express rider.
The object of the preposition of the above sentence is 'pony express rider'.
The preposition is 'about'.
Explanation:
A preposition is a part of speech which tells about the relation between the noun or pronoun with another part of the sentence. In the above sentence the preposition 'about' tells about the relation between 'story' and 'pony express rider'.
