Questions: Merocrine sweat glands are compound alveolar glands. - produce organic secretions that are metabolized by bacteria to produce body odor. - secrete a watery fluid directly onto the surface of the skin. - increase in number and activity with aging. - primarily function in lubricating the hairs.

The answer is the third one: secrete a watery fluid directly onto the surface of the skin.

Explanation for each option:

1. Compound alveolar glands: This is incorrect. Merocrine sweat glands are simple tubular glands, not compound alveolar glands.
2. Produce organic secretions that are metabolized by bacteria to produce body odor: This is incorrect. This description fits apocrine sweat glands, not merocrine sweat glands.
3. Secrete a watery fluid directly onto the surface of the skin: This is correct. Merocrine (or eccrine) sweat glands secrete a watery fluid (sweat) directly onto the skin surface, which helps in thermoregulation.
4. Increase in number and activity with aging: This is incorrect. The number and activity of merocrine sweat glands generally decrease with aging.
5. Primarily function in lubricating the hairs: This is incorrect. The primary function of merocrine sweat glands is thermoregulation, not lubricating hairs.

Summary: Merocrine sweat glands secrete a watery fluid directly onto the surface of the skin, which helps in cooling the body through evaporation.