I could not understand what you wanted me to do.
The underlined part of the sentence is-

The underlined part of the sentence "I could not understand what you wanted me to do" is a noun clause.

Explanation:

• Noun Clause: A noun clause functions as a noun within a sentence. It can serve various roles, such as the subject, object, or complement of a verb. In this case, "what you wanted me to do" acts as a single unit and functions as the direct object of the main verb "understand." It represents the thing that the speaker could not understand.

• Adjective Clause: An adjective clause modifies a noun or pronoun within a sentence. It typically starts with a relative pronoun (e.g., who, which, that). There is no adjective clause in the given sentence.

• Interrogative Clause: An interrogative clause is a type of sentence that asks a question. The given sentence is not a question.

• Adverbial Clause: An adverbial clause functions as an adverb within a sentence, providing information about the manner, time, place, or condition of the main action. The given sentence does not contain an adverbial clause.

Therefore, the underlined part is a noun clause, as it functions as the direct object of the verb "understand."