Adverbial clauses function just like adverbs. In the sentence, the line, when he speaks in public is an adverbial clause. Thus, option b is an adverbial clause.
An adverbial clause is a phrase that acts as same as the adverb and answers the question, when, where, how, what, etc. Here the line about speaking in public answers the question, "when" but does not have a stand-alone meaning.
Therefore, option b. when he speaks in public is an adverbial clause.
Learn more about adverbial clauses here:
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