Which of these is a power that the president of the united states can exercise? a. vote for a bill that is being discussed in congress b. declare a federal law unconstitutional c. overturn an unconstitutional state law d. veto a bill passed by congress?

D. veto a bill passed by Congress

Even though the President can veto a bill, it can be sent back to Congress if they choose to try to overrule the President's veto. In order for this to happen, both the Senate and the House of Representatives must vote to overrule the President's veto by a 2/3's majority. If that happens, the President's veto is overruled and the bill becomes a law.

The President can NOT declare a federal law unconstitutional nor overturn an unconstitutional state law. The Constitution does not grant the President the power to legislate, rather to execute legislation. Neither does the Constitution of the United States grant courts the power to legislate. The power to legislate was given specifically to Congress. The President does have the right to to submit legislation to Congress.
The Constitution does not give the occupant (the President) of that office the authority to "eliminate" a law that has been passed by Congress, no matter what voters think or believe of that law. The process for repealing a law has to begin in Congress.

The President can NOT vote for a bill that is being discussed in Congress because the Constitution does not grant the President the authority of legislation. The President has the option to veto the bill, after passing through both the House of Representatives and the Senate. But the President may choose to sign or veto the bill. If vetoed, Congress may choose to try to overrule it.