Correct Answer:
B. Noun clause
The correct answer is Noun clause. In the sentence 'I can't tell you why I am late,' the underlined portion 'why I am late' is a noun clause. It functions as the direct object of the verb 'tell' and answers the question 'what?' A noun clause acts as a noun in the sentence, while an adverb modifies a verb, a conjunction connects words or clauses, and an adjective modifies a noun. Therefore, it is a noun clause.