The official leader of the US Senate is the President of the United States Senate - but this isn't one of the options.
However, this office is always held by the Vice President of the United States: that's why the correct answer is c.Vice President of the United States.
And in practice, this function is performed by the President Pro-Tempore (but this is not the correct answer, the president Pro-Tempore is only representing the Vice-President)
