Even in her early 60s, Theresa has a full head of dark hair with not a hint of gray. When people ask why, she explains that her mother didn't start turning gray until her mid-70s. Which of the following is the likely cause of Theresa's still-colorful hair?
a. life-cycle forces
b. sociocultural forces
c. biological forces
d. psychological forces

The correct answer to the question is c. biological forces.

Explanation:

This is because Theresa says that her mother also had the same trait of non-greying of hairs till later age.Theresa must have inherited the gene responsible for maintaining dark hair for a long period of time from her mother when she was born.The gene must code for melanin production in hairs as darkening of hair or skin is mediated by the protein melanin.This gene might have been dominantly expressed in Theresa as she is capable of retaining her mother's characteristics till older age.