The primary responsibility for gathering information, taking testimony, and revising legislation in Congress falls to
A. The Conference Committee
B. The Rules Committee
C. The majority party leadership
D. The members as a whole meeting on the floor
E. Standing committees
Generally, the primary responsibility for gathering information, taking testimony, and revising legislation in Congress falls to the standing committees. Once a bill gets introduced into Congress, it is sent to a committee. The committee members (or some members of their staff) do a lot of research on the bill, listen to people who want to speak about the bill, and then the committee members may revise the original bill, accept it as it is written, or decide not to advance the bill. If they don’t advance the bill, this is called killing a bill. If the bill makes it out of committee, then it goes to the full house of Congress for discussion and voting. If the bill was introduced in the Senate, it goes to the full Senate. If it was introduced in the House of Representatives, then it goes to the full House of Representatives.
Please note that some of the other choices listed above may do some of the ideas you are asking in your question. However, it is the standing committees where all of these actions are done most often.