In the mid-1800s, travelers faced the greatest risk of attack by Native American groups on the California Trail.
Mormon Trail.
Oregon Trail.
Santa Fe Trail.

Santa Fe Trail.

Explanation:

The Santa Fe Trail was a 19th-century transportation route through central North America that linked Franklin, Missouri with Santa Fe, New Mexico. The route of the Santa Fe trailed crossed the territory of the Comanches, the north-western corner of Comancheria which is a Native American nation.

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering

i also think it is c