Here's why:
* Preposition: The preposition in this sentence is "at".
* Object of Preposition: The object of a preposition is the noun or pronoun that follows the preposition and answers the question "what?" or "whom?".
* In this case: The phrase "at ten" tells us the time of the meeting. The noun "ten" directly follows the preposition "at" and answers the question "at what?".
Therefore, "ten" is the object of the preposition "at".