My internal reasoning for accepting JY's rationale for excluding option A is that option A is merely an extension of premise 2. This distinction matters because if you assume that "activists believe the proposal shouldn't pass," it is a plausible statement, but it shifts the conclusion of the stimulus from "at least one member of the city council should vote against the proposal" to "the proposal shouldn't pass." While we may intuitively process these as equivalent, there could be underlying reasons why activists do not want the bill to be voted on by the public while still believing it shouldn't pass. For example, publicly deciding on the proposal could backfire on them for strategic or political reasons.

Returning to an LSAT-style explanation, strictly analyzing the relationship between the premise and conclusion, the premise is only activated when we assume that the bill should not be voted on by the citizens.