Which one among the following is present in the nettle leaf hairs that causes burning pain?

Nettle (Urtica dioica) leaves have stinging hairs (trichomes) that inject formic acid (methanoic acid, HCOOH) into the skin upon contact, causing a burning sensation and pain. Along with formic acid, the hairs also contain histamine and acetylcholine, but methanoic acid is the principal irritant. This is the same acid found in the stings of ants and bees. Ethanoic acid (acetic acid) is found in vinegar, and benzoic acid is used as a preservative.