The correct answer to the question given above is letter b, adverb phrase.
In the sentence "Few will master all the steps in one week.", (whose underlined phrase is one week) the phrase "one week" is an adverb phrase.
>>An adverb phrase is simply a word or group of words that act as an adverb. It can modify a verb, adverb, or adjective and can tell “how”, “where”, “why”, or “when.”
