The sac that regulates the temperature of the testes is the rectum bladder scrotum urethra seminal vesicle

Scrotum

Explanation:

The male reproductive organ called TESTIS (plural- testes), which produces the sperm (male gamete), is enclosed in a bag-like structure called SCROTUM. The scrotum which is found between the thighs in males, helps regulate the temperature of the testes.

N.B: A lower/cooler temperature than the normal body temperature (37°C) is needed for the testes to produce sperm (spermatogenesis).

The scrotum keeps the temperature of the testes right for sperm production by adjusting its size in response to temperature. When the body temperature is cold, the scrotum contracts or become smaller in order to hold body heat. When the body temperature is warm, the scrotum becomes flabby and loose in order to release excess heat.