Final answer:
In the U.S., political parties must strive to become broad-based coalitions or face being irrelevant. This is due to the winner-take-all system of elections and the need for clear party platforms.
Step-by-step explanation:
The subject of this question falls under the category of Social Studies for High School students.
U.S. political parties must strive to become broad-based coalitions or face being irrelevant, in part because of the winner-take-all system of elections. In a winner-take-all system, the candidate who receives the most votes wins all the power, which makes it difficult for third parties to gain traction.
By contrast, a proportional system of elections would allow voters to pick their preferred party, and legislative seats would be allocated to competing parties based on the total share of votes they receive in the election. This could give smaller parties a chance to be represented in government.
Therefore, the correct answer is a) winner-take-all system of elections and b) proportional system of elections.