Pioneers who journeyed to the west moved quickly through mountain passes to avoid traveling
a-through rainstorms, which made trails slick and wet.
b-in the dark, which made it difficult to follow the trail.
c-in large groups, which could attract american indian attacks.
d-through the first snows of winter, which could trap a party.

The answer is "D", through the first snows of winter, which could trap a party.

It was essential to cross the plains in Summer so they could get through the Rocky Mountains and arrive in California before the winter snows closed the mountain passes. The ones unable to make it through were stranded without enough provisions for the entire winter.

I believe that the answer is D. through the first snows of winter, which could trap a party.

maybe i can