Answer:
A subject
Explanation:
A clause is a group of words that have a subject and a verb. And an independent clause is a clause that expresses a complete thought, and therefore, it can stand alone as a full sentence.
"threw the ball" has a verb: "threw", therefore, what it is missing is a subject like "The baseball player," "Kevin" or "He". If you add any of those subjects, or whichever subject you can come up with, the clause, then, would also express a complete thought and would be an independent clause:
"The baseball player threw the ball"
