What was the primary motivation for most settlers in the western territories in the 1800s

The promise of free land.

Explanation:

Industrialization had not taken roots in the 1800s.

The main economic venture was farming which needed a land as a primary resource.

The discovery of new western territories led to caravans of settlers approaching to get the free land resource.

This makes the third option right.

c

explanation:

the passage of a strict fugitive slave law