13. This type of region is unified by a symbiotic relationship, which means all the places Work together for a common purpose and benefit from each other.
A. Natural
B. Perceptual
C. Formal
D. Functional

D. Functional

Explanation:

A symbiotic relationship is a type of relationship where two or more parties benefit from each other mutually.

The type of region that is unified by a symbiotic relationship is a functional region because each organism benefits.

answer; (d) melting of sea ice;