Question

an example of a sensor in a negative feedback loop that controls body temperature is
skeletal muscle contraction
a thermoreceptor
the hypothalamus
a sweat gland

Explanation:

In a negative feedback loop for body temperature control, a sensor detects changes. Thermoreceptors detect temperature changes (input). Skeletal muscle contraction and sweat glands are effectors (output actions). The hypothalamus is the control center, not a sensor. So a thermoreceptor is the sensor.

Answer:

a thermoreceptor