The graduate students filled out the necessary applications, and submitted the required references.

how is the comma used incorrectly in the sentence above?

the comma attempts to separate a compound predicate.
the comma attempts to set apart an essential element.
the comma attempts to separate a compound subject.
the comma attempts to join two independent clauses.

The answer would be A: "The comma attempts to separate a compound predicate." This is due to the fact that a compound predicate means that there are two actions for one subject which means that there is more than one verb in the sentence. The two actions or verbs in the sentence would be that the graduates fill  out all the necessary applications and then that the graduates also then they submit them.

Explanation:

sight: watching a movie

touch: petting a dog

smell: smelling dinner

hearing: listening to music

taste: eating pizza

i think the answer is d.