Biology Review physical or chemical influence?

Melanin pigments lie under the surface of the skin. Exposure to sunlight increases the production of melanin to protect the skin from UV-rays.

The fact that exposure to sunlight increases melanin production in many humans and produces a tan demonstrates that

a.the environment causes mutations that have no effect on the chance of survival
b.the environment causes mutations that have no effect on the chance of survival
c.the environment causes mutations that increase the chance of survival
d.the expression of some genes is influenced by the environment

HINT: IS this a physical or a chemical influence on DNA molecules?