When parties only win pluralities, not majorities, those parties don’t effect change in their country’s political arrangements. Therefore, the victories of the extremist parties Chapin discusses pose no threat to democracy in their countries. (A) validates Chapin’s conclusion.

Chapin doesn’t differentiate between multiparty and two-party political systems. Chapin only argues that the victories of the extremist parties in his argument pose no threat to democracy in their countries. (B) is irrelevant.

Chapin’s argument is about specific countries where extremist parties have recently won elections. It’s not clear whether or not (C) is even discussing the countries in Chapin’s argument, so (C) is irrelevant.

Chapin only discusses countries where moderate parties were preoccupied with arguing among themselves, so (D) is irrelevant.

The reason people support extremist parties is irrelevant. Chapin’s argument is only concerned with whether the recent electoral success by extremist parties in several democratic countries poses a threat to democracy in those countries. (E) is irrelevant.