Which part of speech has been used in the bold word in the sentence? “The plane goes direct from London to Houston without stopping.”

The bold word is direct, and it is an adverb. It tells us the manner or route of the action: the plane travels in a straight line with no change of flight. It modifies the verb goes, and an adverb answers questions such as how, where or to what extent an action happens.

• Noun (A): Wrong. The word names no person, place or thing, and no determiner such as "a" or "the" comes before it.
• Adjective (B): Wrong here. In "a direct flight" it is an adjective because it qualifies the noun flight. In this sentence it qualifies the verb, which is the adverb's job.
• Conjunction (D): Wrong. A conjunction joins words, phrases or clauses, and "direct" joins nothing.

The core concept is the flat adverb: some adverbs share their form with adjectives, as in "go direct", "come straight" and "drive slow". In Mains, this matters for precis and for correcting usage such as "drive slow" versus "drive slowly". Always ask what the word modifies before you name its part of speech.

Trap: Because "direct" looks like the adjective in "direct flight", many candidates label it an adjective without checking whether it modifies a noun or a verb.