The given sentence contains three distinct phrases. The first is a noun phrase 'Those men in the clinic' as it exemplifies a group of words that together functions as the subject in the sentence. It is followed by the verb phrase 'are serving' which comprises of the auxiliary and the main verb to collectively convey the action of the subject. Lastly, the adverbial phrase 'as front-liners' function to modify the action of the verb 'serving.' Thus, <u>option b</u> is the correct answer.
