In the sentence ‘I knocked the door but nobody answered,’ the word but is a:

The correct option is A. Conjunction. In the sentence 'I knocked the door but nobody answered,' the word 'but' serves to connect two independent clauses ('I knocked the door' and 'nobody answered') and introduces a contrast between them. This is the primary function of a conjunction, specifically a coordinating conjunction.

• B. Preposition: A preposition shows the relationship between a noun/pronoun and another word in the sentence (e.g., 'on the table', 'with a friend'). 'But' does not function this way here.
• C. Adverb: An adverb modifies a verb, an adjective, or another adverb (e.g., 'quickly ran', 'very happy'). 'But' does not modify another word in this sentence.
• D. Interjection: An interjection is a word or phrase that expresses strong emotion (e.g., 'Oh!', 'Wow!'). 'But' is not used as an interjection here.