The squirrel reached for the birdfeeder, for he saw it was filled with an abundance of sunflower seeds. the word for in the sentence is used as both a-a gerund and an adverb. b-a noun and an adjective. c-a preposition and a conjunction. d-a verb and a demonstrative article.

The word for in the sentence is used as both C. a preposition and a conjunction. 
A isn't correct because a gerund is a verb-like form that ends in -ing. B isn't correct because for is always used as either a preposition or conjunction, it can never be a noun or adjective. D isn't correct because there is no such thing as a demonstrative article, there are only three articles: indefinite a/an, and definite the.