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