Of these choices: carotene, hemoglobin, melanin, what is the most responsible for the skin color of dark-skinned people? What provides a natural sunscreen?
Melanin is responsible for skin color, carotene can provide some protection against the sun.
Melanin is a pigment that gives the skin color. This melanin is produced by so called melanocytes. Melanocytes are located at the bottom of the top layer of the skin (the epidermis). Melanocytes of people with dark skin produce more melanin.
Melanin is the bodies own way to protect the skin against sunlight. The molecule effectively absorbs UV-light and neutralizes damaging molecules (radicals) created by exposure to sunlight.
Carotene or rather
Hemoglobin is the protein in red blood cells that gives the blood a red color due to the interaction of iron. When your skin is red this can be caused because more blood flows to the area. This is further not relevant in this question.