The answer is A. Authority. Based on the meaning from the Merriam-Webster, authority is "the power to give orders or make decisions".
As for veto, it is "a decision by a person in authority to not allow or approve something (such as a new law)".
And for majority, "a number of votes that is more than half of the total number".
This gives us the differences between the three, and thus authority is the power to give final decisions and not veto nor majority.