Final answer:
Hair turning gray is commonly caused by the natural aging process, due to a decrease in melanin production in the hair follicles.
Step-by-step explanation:
The process of hair turning gray is most commonly caused by a) the natural aging process. Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla.
As a person becomes older, melanin production in the hair follicles decreases, and their hair tends to lose its original color, leading to gray or white hair. This change in hair color can be regarded as one of the physical markers of age.