What Does the "Inverted U-Curve" Describe

What does the "inverted u-curve" describe?

A) Political parties in the centre of the left-right spectrum generally support European integration, while those at either extreme tend to oppose it.
B) Electorally successful parties generally support European integration, while unsuccessful ones oppose it.
C) Parties that are left on the economic dimension tend to be on the right on the social dimension.
D) Ideologically extreme parties get less votes than parties in the ideological middle.