The correct answer is option 4, blood capillaries. Blood capillaries are small, thin-walled vessels that are found throughout the body, including the skin. These capillaries help in regulating body temperature by dilating or constricting in response to external temperatures. When the body is exposed to cold temperatures, the blood vessels constrict, reducing blood flow to the skin and minimizing heat loss. On the other hand, when the body is exposed to hot temperatures, the blood vessels dilate, increasing blood flow to the skin and facilitating heat loss through the process of radiation. This helps in keeping the body warm or cool depending on the external conditions. The other options, including pigmented cells, sweat glands, and lymph vessels, do not directly contribute to regulating body temperature in the same way as blood capillaries.