To constitute a majority in the U.S. Senate, you would need more than half of the 100 senators. Half of 100 is 50, so a majority would be 51 senators.
Therefore, the answer is: 51 (though this option isn't listed among the responses).
If you're limited to the options given and have to choose one, the correct option would be 75 since it's the only one greater than half, but keep in mind that the actual majority number is 51.