Which word fits in this context: "The leader of the trade union tried ____________ for re-election before finally retiring after 27 years."?

In this context, the sentence describes the leader's attempt to run for re-election. The correct answer, "unsuccessfully," operates as an adverb, which modifies how the action of trying for re-election is performed. Here, it indicates that the leader did not succeed in securing re-election. The use of "unsuccessfully" effectively communicates that, despite the leader's efforts, they ultimately did not win another term before retiring.

The other words do not fit grammatically or contextually. "Successful" is an adjective and would need to modify a noun, while "success" is a noun and cannot appropriately complete the action of trying. "Successfully," although an adverb, would imply that the attempt was fruitful, which contradicts the context of the leader's retirement following a lack of re-election.