The word in the sentence in which the adverb clause modify is letter A: helmet.
Adverb clauses or phrases are group of words used in shifting or adding meaning of a verb, clause, adjective, or another adverb. These clauses can be used if: firstly, a sentence contains a subject and a predicate. Secondly, when a subordinate conjunction is used and thirdly, if it answer the ‘adverbial questions’ – when, why, when and how. It is also further categorized to time, place, manner, degree, condition, concession and reason.