Answer:
C. Third parties serve to strengthen the two main political parties.
Step-by-step explanation:
So, basically, 3rd parties will rarely win. However, because they exist, they still take votes away from the two main parties. While this, at first, seems like it would weaken the two main parties, While this, at first, seems like it would weaken the two main parties, their existence ensures that the two main parties actually try to improve and add to their agenda.
I hope this helps, have a blessed day. :D