The correct answer is B
Congress has two houses: the senate and the house of representatives. The Answer A, president pro tempore of the Senate is wrong- this person is not even the most important person in senate, only the second most important. In fact the highest person in the senate is the Vice President of the United States. On the other hand, the highest person in the house of representatives is the Speaker of the House. So we know it's either B or D. Now, the question is tricky, as neither of the chambers of the senate is officially more importnat. Additionally vice-president is higher in the line to the succession to the President's office.
However, it is usually assumed that the Speaker represents the Congress as a whole, and that's why i'd go with B.
