Answer:
B. Political ties
Step-by-step explanation:
People in Congress usually get to sit on the most interesting committees because of political ties.
In the political landscape in Washington and in many parts of the world, politics is a "scratch my back and I scratch yours" game which involves doing favors for someone so they would in turn do a favor for you sometime.
Therefore, giving members of Congress an opportunity to sit on the most interesting committees is based on political ties and favors.