It's is an adjective phrase as an adjective describes a noun and the noun is a sweater
E I believe I may be wrong, hope this helps.
An adjective clause is a type of clause that also contains a subject and a verb, but functions to describe a noun. This clause is often introduced by pronouns such as whom, who, that, which and the adverb where, when and why. In the given sentence above, the adjective clause is, "who received a good grade". This clause describes the noun "student".
