What part of this speech is the word roundup in sentences Today they were going on a roundup?

The word "roundup" in the sentence "Today they were going on a roundup" is the object of the preposition "on".

Here's why:

* Prepositional Phrase: The phrase "on a roundup" acts as a prepositional phrase, modifying the verb "going."

* Preposition: "On" is the preposition.

* Object of the Preposition: The noun phrase "a roundup" follows the preposition and tells us what they are going "on."