• Noun clause: A noun clause functions as a noun in a sentence, acting as the subject, object, or complement of a verb. In the given sentence, "why you did this" is the object of the verb "tell". If we ask "Tell me what?", the answer would be "why you did this". 
• Why you did this: This clause expresses a reason, which is a noun-like function. 

Why other options are incorrect:

• Adjective clause: Adjective clauses modify nouns or pronouns, usually beginning with relative pronouns like who, whom, which, that, or whose. The underlined part doesn't modify any noun or pronoun. It functions independently as a noun clause. 
• Adverbial clause: Adverbial clauses modify verbs, adverbs, or other clauses, indicating time, place, manner, reason, condition, concession, or consequence. The underlined part doesn't modify a verb, adverb, or other clause. It functions as a noun clause. While it does express a reason, the why doesn't modify anything. It's the entire clause that functions as the reason. 
• Adverbial phrase: Adverbial phrases modify verbs, adjectives, or other adverbs, indicating time, place, manner, degree, frequency, purpose, or other circumstances. The underlined part is not a phrase; it's a clause with its own subject and verb. An adverbial phrase would be something shorter, like "quickly" or "in the park".