Structures in the dermis that regulate body temperature are
a. oil glands and sweat glands.
b. blood vessels and sweat glands.
c. muscle fibers and blood vessels.
d. oil glands and blood vessels.

The correct answer is b. blood vessels and sweat glands.

Explanation:

Blood vessels and sweat glands are mostly responsible for the regulation of body temperature. When the body temperature becomes low the blood vessels and capillaries present under the skin get shrinks that reduce the blood flow which results in heat loss reduction and vice versa happens when body temperature rises too much.

Sweat glands are present everywhere in our body and it release sweat which contains water and salt. When body temperature rises too much body release sweat and its evaporation cause cooling of the body.  

Therefore blood vessels and sweat glands are the structures in the dermis that helps in thermoregulation.