Which of the following sentences about Gary is grammatically correct?

The first sentence is grammatically correct because it properly employs the relative pronoun "who," which is used for people. In this context, it correctly refers to Gary, indicating that he was sorry about not using Study.com while also conveying that he believes in the value of higher education.

The structure includes a non-restrictive clause (the phrase that adds information about Gary) set off by commas. This indicates that the extra information about Gary's feelings does not limit or change the subject's recognition—it's just additional context.

In contrast, using "that," "which," or "whom" in other options does not fit correctly with the subject and purpose of the sentence, as "that" is typically used for things, "which" is also for things or non-human entities, and "whom" is generally used to refer to the object of a verb or preposition. Hence, these formulations do not correctly align with the role of 'Gary' in the sentence, resulting in grammatical inaccuracies.