Final answer:
The contraction of the dartos muscle tightens the scrotal sac, which is a thermoregulatory response to keep the testes at an optimal temperature for sperm production and maturation.
Step-by-step explanation:
The contraction of the dartos muscle tightens the scrotal sac. This action occurs as a response to cold temperatures and is aimed at moving the testes closer to the body to retain heat, thereby decreasing the surface area of the scrotum. In contrast, when the environmental temperature increases, the scrotum relaxes, allowing the testes to move farther from the body core which increases the scrotal surface area and promotes heat loss. The dartos muscle does not produce an erection, propel sperm through the urethra, move sperm through the ductus deferens, or initiate seminal emission. Therefore, the correct answer to the student's question is A) tightens the scrotal sac.