Answer:
People should prepare for the worst (when the weather service predicts a
hurricane.)
Explanation:
The sentence in which the underlined adverbial clause is punctuated correctly is option A.
An adverbial clause is a dependent clause which can also perform the functions of an adverb by modifying a verb, adjective or another adverb in a sentence.
The other two options are incorrect because they were missing either a comma or a semi colon and as such were not properly punctuated.
