Answer:
To keep warm during the winter months.
Step-by-step explanation:
The Plateau Indians are the groups of Native Indians comprising of different tribes that reside in the high plateau areas in between the Rocky Mountains and the coastal mountains. They reside here because of the presence of lakes, trees, forests, rivers, etc. for their easy survival.
The Plateau Indians dug pit houses to keep themselves warm from the extremely cold weather during the winters. While the summers are hot during which they live in teepees, winters are of the extreme level, which is why they dug pit houses, lining underground to keep warm.
Thus, the correct answer is the first option.