Yesterday they visited an art museum.

is that base, past, past particle, or present particle?

Past

Explanation:

In the given sentence Past tense is used.

Here the subject visited an art museum yesterday. Hence it was an completed action and it falls under past tense category.

Here the subject completed the action in a definite time in the past.

Here the verb is in the past form of "visit". Hence from the above statements it is clear that Sentence type belongs to PAST.