Is this sentence an adverb to consider the proposal?

It is not a sentence, but an infinitive phrase -- the infinitive verb "to consider" followed by the noun "proposal." Depending on how it appeared in a sentence, it could be either a noun phrase, adjective phrase, or adverb phrase.

Their only option was to consider the proposal. (noun)

The decision to consider the proposal was controversial. (adjective)

The board met in special session to consider the proposal. (adverb)