The correct answer is (D) Ringleader, one who assumes the leadership of a group, profession, industry, etc.; someone who takes the lead; a leader of a flock. Also, an indicator of trends. Finally, a wether (castrated male sheep) or other male sheep that leads the flock, usually wearing a bell.

Bellwether comes from Bell and Germanic wethruz (castrated ram).

Examples of use:

• That district is considered to be a bellwether riding during national elections.
• The United States is a bellwether for young people's clothing fashions.