Answer:
The student who had not studied the lessons failed the test.
Subordinate clause: who had not studied the lessons
Subordinate clause type: adjective
Step-by-step explanation:
A subordinate clause is a group of words containing a subject and a verb that does not express a complete thought on its own, and therefore it can not stand alone as a full sentence. They can act as adjectives, adverbs or nouns in a sentence.
When a subordinate clause acts as an adjective, it is placed right next to the noun it modifies, and it usually begins with a relative pronoun (who, whom, whose, that, or which) or a relative adverb (when, where, or why).
The clause "who had not studied the lessons" has all those characteristics: it is a subordinate clause because it does not express a complete thought on its own and it has a subject and a verb, and it is an adjective clause because it is placed beside a noun "the student", its function is to modify or adds further description to that noun, and it begins with the relative pronoun "who."