Answer:
c. mid-adulthood
Explanation:
This stage of life when gray hairs become overwhelming in number and he has more gray hairs than brunette( brown hair) is called middle adulthood. Middle adulthood spans from 20 to 40 years of life which may be defined differently for different culture and country.
Robert is in his late mid adulthood somewhere near 40 at this age hair generally grow grey.
