why is it B? not C

I think the key difference between B and C is that b claims that parties only work to understand what positions are popular with what groups, while c claims that parties actively organize and cultivate groups to press for support of issues. The latter is not really described as what parties do in the passage. The passage tells us in the second paragraph that parties work to understand what positions they can adopt to win over specific groups, but it doesn't say that they organize and create those groups.