What important role do third parties often play in us national elections? a) causing major parties to fight one another b) breaking up major parties’ dominance c) challenging people with more choices d) generating malice for larger parties

Short Answer B
If the major party is confronted by a minority that has split away and put itself on the ballot, the people who would normally vote for that party now have 2 choices and they will choose the one that most closely aligns to their own beliefs. 

It happens much more often in Canada and some European countries and places like Israel than it does in the United States. I live in Alberta Canada and I can tell you a more conservative place does not exist anywhere else in Canada. There were 2 conservative parties in the last election and both of them were a good choice if you were a conservative. That's the problem. The New Democratic Party (a socialist leaning party) swept the province with a huge majority of seats. They got this majority with only 41% of the popular vote. 

The vote broke down this way
NDP  41%
Conservative party #1  27%
Conservative party #2  25%

The remainder were other parties. 

The answer is B