The correct answer is Adverb.
An adverb is a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. (e.g., gently, quite, then, there ).
An adjective is a word naming an attribute of a noun, such as sweet, red, or technical.
A verb is a word used to describe an action, state, or occurrence, and form the main part of the predicate of a sentence, such as hear, become, happen.
A preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause, as in ‘the man on the platform’, ‘she arrived after dinner, ‘what did you do it for ?’.
Here, past is an adverb as it modifies the verb pushing.