According to the study a person with a darker complexion is more protected from UV rays compared to a person with fair complexion. Explain why.

A person with a darker complexion has more melanin in the skin. Melanin is the component responsible for dark pigmentation and the one that naturally protects us against  UV rays. So, a person with a darker complexion will be more protected from UV rays than someone with a fair complexion.

Explanation:

People with darker complexion have higher levels of melanin in the epidermis. Melanin is a natural pigment that, besides giving darker complexions, when it is in high quantities, protects us against the UV rays damage, blocking some of these rays, but not as efficiently as sunscreen. As people with darker complexion have more melanin, they also have more protection against UV rays than people with fair complexion.

not concrete, existing in thought or as an idea but not having a physical or concrete existence

"a" because he is appealing to logic as oppose to appealing to the audience's emotions.