How did fur traders help in the westward expansion of the United States?

The trails they established to move their furs east were later used by pioneers.

Explanation:

In that time period, the map of United States was unclear. So it's hard for people to reach different states without being lost. During the trades, fur traders often have to create their own camp to take a rest in-between their journeys. They left a lot of objects there after they've finished their rest.

This is why These resting places created 'Trails' that other people can follow to reach the western part of United States. The pioneers who were involved in the westward expansions use the traders' trail to reach their destination.

all of them are correct