Concept:The correct word describes the difference by which an election result is won.
Explanation:A margin is the gap between the winner and the loser in an election.
The sentence refers to this exact difference in votes.
"Surplus" means an extra amount, and does not fit here.
"Consensus" means general agreement, which is not the meaning needed.
"Space" refers to an empty area, so it is unrelated to election results.
Thus, "margin" is the only word that logically completes the sentence.
Answer:D. margin