Final answer:
The adjective dependent clause in the sentence is "that you attended", which describes the noun "high school" and contains a subject and a verb.
Step-by-step explanation:
The adjective dependent clause in the sentence "The high school that you attended must have had a wonderful English program" is "that you attended". This clause is functioning like an adjective because it describes the noun "high school", specifying which high school is being talked about. Dependent clauses cannot stand alone as a full sentence and typically begin with relative pronouns like "that", "which", or "who", or with subordinating conjunctions such as "because", "since", or "although". In this example, "that" is the relative pronoun introducing the dependent clause, which contains a subject ("you") and a verb ("attended").