A check on the power of the legislative branch means that the legislative branch is the one being stopped. There can be two ways that a bill that is passed by the legislative branch can be stopped:
1) A veto against the bill (Option A)
2) The Supreme Court rules that the law is unconstitutional.
Note that for option (C), while it states the Supreme Court, it is talking about presidential actions (executive branch powers), and so it is not the answer
Option A is an example of a check on the power of the legislatibe branch of the constitution. The president can veto a legislation thanks to the existence of Checks and Balances, which funtion is to make sure no one branch would be able to control too much power, that is that this system limits the power of the three branches devided by the constitution: Legislative, executive and judicial. In this case the executive (President) is limiting the legislative power (Congress)
