Question

Which of the following is NOT a check that the Legislative Branch has over the Executive Branch?

A. can impeach the president
B. approves presidential nominations
C. declare presidential acts unconstitutional
D. controls the budget

Answer 1

The Legislative Branch cannot "declare presidential acts unconstitutional," since this is one of the ways in which the judicial branch checks power, through the act of "judicial review". 

Answer 2

C. Declaring presidential acts unconstitutional is NOT a check that the Legislative Branch has over the Executive Branch.
The Judicial Branch is the branch of government that has power to declare a presidential act unconstitutional. The Judicial Branch also exercises that check over against the Legislative Branch, having authority to declare a law passed by Congress as unconstitutional.
The other three items listed -- budget, approval of nominations, and impeachment -- all are powers the Legislative Branch can exercise as a check and balance on the Executive Branch.