We can answer that question in several ways:
1. "Sundays" is a proper noun. That means that "Sundays" is a noun that we always capitalize because it stands for a certain special thing.
2. "Sundays" is a plural noun. It means more than one Sunday--in this case, probably every Sunday.
3. "Sundays" is the object of a preposition. In the prepositional phrase "on Sundays," the word "on" is a preposition, and the word "Sundays" is the object of that preposition.
In every case above, "Sundays" is a noun: a thing, and not, say, an action.
My best guess is that you need to determine the role that the word "Sundays" plays in the sentence: its part of speech, which is a noun.
In case you need a little more information, let's take another look at the sentence and its sections:
"Thom usually visits his grandparents on Sundays."
In that sentence, "Thom" is the subject, and "usually visits his grandparents on Sundays" is the predicate.
We can get more specific by saying that "Thom" is the subject, that "visits" is the verb," that "his grandparents" is the direct object of the verb, and, again, that "on Sundays" is a prepositional phrase.
Just in case you need to identify the parts of speech for the other words in the sentence, let's list those, too:
Thom = proper noun
usually = adverb
visits = verb
his = possessive pronoun (it works like an adjective to tell more about the noun "grandparents")
grandparents = common noun
on = preposition
Sundays = proper noun