In the sentence, “he finished the job at one go”, the word ‘go’ is

The phrase 'at one go' means in a single attempt or without stopping, so 'go' here names that attempt. It works as a noun, which makes B correct.

• Verb (A): 'Finished' is the verb of the sentence. 'Go' sits after the preposition 'at', and a word in that position functions as a noun, not a verb.
• Adverb (C): An adverb modifies a verb or adjective, as in 'he worked quickly'. 'At one go' is a prepositional phrase, and 'go' is its object, so it cannot be an adverb.
• Adjective (D): An adjective qualifies a noun directly, as in 'a quick job'. Here 'go' stands alone as the object of 'at' and qualifies nothing.

Core concept: grammar is decided by the word's role in the sentence, not by its dictionary form. 'Go' is also a noun in 'have a go', so the same word can shift class. Such idiom-based questions test whether you read the structure. This also helps in Mains essay and precis writing, where precise idiomatic use shows command of English.

Trap: Aspirants see 'go' as the verb 'to go' and pick A. Ask what the word does in the phrase, not which verb it looks like.