Final answer:
The reddish color of the lips is primarily due to hemoglobin, which provides a pinkish tint visible through the thin skin of the lips, particularly when blood flow is increased in the capillaries of the underlying dermis. So the correct option is a.
Step-by-step explanation:
The reddish color of the skin of the lips is primarily due to hemoglobin, a red pigment found in red blood cells. The pink color is most visible in the lips where capillaries in the underlying dermis dilate, increasing the blood flow and allowing the color of the hemoglobin to be seen through the thin layer of skin on the lips.
While melanin is the main pigment responsible for skin color, and carotene can give skin a yellowish tint, it is hemoglobin that gives the lips their characteristic red color. This pigment is more visible in the lips due to the lower levels of melanin and the thin skin that covers the area.