The correct answer is option 2: Methanoic acid.

When the leaves of a nettle plant are touched, they can cause a burning sensation on the skin. This sensation is due to the injection of methanoic acid, also known as formic acid, into the skin.

Methanoic acid is a naturally occurring compound found in the stinging hairs of nettle plants. These hairs, called trichomes, contain tiny needle-like structures called spines. When the spines come into contact with the skin, they break off and release methanoic acid.

Methanoic acid is a strong acid and can irritate the skin. It causes a stinging or burning sensation and can also lead to the formation of small red bumps or welts on the skin. The sensation can vary in intensity depending on the individual and the amount of contact with the nettle leaves.

It is important to note that despite its unpleasant effects, nettle leaves and their stinging hairs can also have medicinal benefits when prepared and used properly. They have been used in traditional medicine for their anti-inflammatory and pain-relieving properties. However, caution should be exercised when handling nettle leaves to avoid any unwanted contact with the stinging hairs.