Explanation:

- The foul smell from a body, especially during a hot summer, is mainly due to the action of bacteria on sweat.

- Bacteria thrive in warm, moist environments. When they break down sweat, a foul odor is produced.

- Melanin is a pigment that determines skin and hair color. It is not involved in sweat odors.

- Moisture itself does not cause smell. It provides a suitable environment for bacteria that cause odor.

- Viruses do not produce smells. They are unrelated to body odor from sweat.

Option 3: bacteria