This depends on the sentence given. Prepositions are words that associate an object to a certain location or position. Examples of prepositions are: on, after, below, by, during, and many more. If you want to use the word 'opportunities' as the object of your preposition, an example would be as follows:
Mary is hopefully praying for good luck as she takes on opportunities. The preposition is 'on' and the object of preposition is 'opportunities'.