The correct answer is D) special group of people that mediate party-related problems.
The importance of moderates in the political process is that they are a special group of people that mediate party-related problems.
The problems with radicals are that they are in the extremes of any situation. Their beliefs are extreme and often show a radical position on issues. So if you have two radical postures, there is no movement. The issue comes to a halt. That is when the moderates come in with a more reasoned, less passioned point of view. They can place arguments in the middle of two extremes and that helps things move again.