Which factor plays the greatest role in determining the climate in a highland region?

Elevation.

Explanation:

Elevation plays an important role in determining the climate in a highland region.

The climate of highland regions becomes cooler and colder as the elevation increases. This factor also affects rainfall patterns in mountainous and highland region.

Above timberline, no trees grow but the area above timberline on highlands or mountainous region is covered with snow.

So, elevation is an important factor.