The correct answer is option 3: thermoregulation. Sweat glands in mammals play a crucial role in regulating body temperature. When the body becomes too hot, the sweat glands produce sweat, which is mostly water along with some salts. As the sweat evaporates from the skin`s surface, it helps to cool down the body. This process is known as evaporative cooling and is a vital mechanism for maintaining a stable body temperature in mammals.

Option 1: Removal of excess salts is not the primary function of sweat glands. While sweat does contain some salts, its main purpose is to cool the body, not specifically remove salts.

Option 2: Excretion of nitrogenous wastes is not the primary function of sweat glands. Nitrogenous waste excretion is primarily carried out by the kidneys and not sweat glands.

Option 4: Sweat glands are not directly involved in sex-attraction.