The correct answer is option 3, ant stings. Methanoic acid, also known as formic acid, is a naturally occurring substance found in the venom of certain ants. This acid is responsible for the stinging sensation and irritation caused by ant bites. When an ant stings, it injects methanoic acid into the skin, which elicits a defensive response or serves as a deterrent to potential threats.

Option 1, muscles, is incorrect. Methanoic acid is not typically found in muscles, as it is primarily associated with certain species of ants.

Option 2, urine, is also incorrect. While formic acid can be detected in trace amounts in urine, it is not a normal or major constituent of human urine.

Option 4, human brain, is also incorrect. Though formic acid is naturally present in the human body, it is not typically found in significant amounts in the brain.

Therefore, the correct answer is option 3, ant stings, as methanoic acid is normally found in the venom of certain ants.