Answer: The sentence that contains a correctly identified compound object in a prepositional phrase is "We were supposed to go to the bank and grocery store".
Explanation: The object of a phrase is compound when it consists of two or more nouns/pronouns. In the sentence provided, the prepositional phrase is "to the bank and grocery store" since the phrase has been introduced by the preposition "to". The object of this prepositional phrase is "the bank and grocery store". In that way, it is a compound object because it is made up of two nouns: "bank" and "grocery store", which is a compound noun. In contrast, all the other sentences lack a compound object in a prepositional phrase.