The correct answer is option 1, Methanoic acid. Stinging hairs of nettle leaves contain a mixture of chemicals that can cause a painful sensation when they come into contact with the skin. The main chemical responsible for the burning pain is methanoic acid, also known as formic acid.

Methanoic acid is a colorless liquid with a pungent odor. It is found in the venom of stinging ants, bees, and wasps, as well as in the stinging hairs of nettle leaves. When the hairs of nettle leaves come into contact with the skin, they inject the fluid containing methanoic acid into the body. This acid irritates the skin and causes a burning sensation.

It`s important to note that other chemical compounds present in the fluid from nettle leaves may also contribute to the pain. However, methanoic acid is considered to be the primary culprit for the burning sensation.

Therefore, option 1, Methanoic acid, is the correct answer as it accurately represents the fluid injected by the stinging hairs of nettle leaves.