Question

it was after midnight, and there was loud knocking on the door.
gerund
infinitive
participle

Explanation:

In the sentence "It was after midnight, and there was loud knocking on the door", the word "knocking" is a gerund. A gerund is a verb form that functions as a noun, ending in -ing. Here, "knocking" acts as the subject of the clause "there was loud knocking on the door", functioning as a noun (referring to the action of knocking). An infinitive is usually in the form "to + verb", and a participle (present participle) would be used as an adjective or part of a verb phrase, but here "knocking" is a noun - like entity (gerund) rather than an adjective or part of a verb phrase in the traditional sense of a participle, and it's not an infinitive.

Answer:

gerund