The comma before “that” is unnecessary. A common error on
SAT/ACT/EST punctuation questions is to randomly place a comma or semicolon
within a sentence, especially in sentences in which there are other commas. In
this sentence, answer choice D is the correct answer. No
punctuation is needed. Remember that if punctuation isn’t needed, you should
leave it out.
In Example 1, the
phrase who works hard does NOT narrow down who we're talking about. We already
know it's just one person—Jonathan—and he happens to work hard.
In Example 1, the
phrase who work hard narrows down the scope of who we're talking about.
Otherwise, the sentence would be talking about ALL students. It's only the
students who work hard who will ace the ACTs.