The correct option is Methanoic acid.
Explanation
Various organic acids occur naturally in plants and animals, often serving as defense mechanisms. The burning sensation caused by an ant sting is due to the injection of a specific carboxylic acid into the skin.
Option Analysis:
- Methanoic acid is correct: Methanoic acid, commonly known as formic acid ($HCOOH$), is the acid present in an ant sting. It is also found in the sting of nettle leaves. The injection of this acid causes pain and irritation.
- Ethanoic acid is incorrect: Ethanoic acid, commonly known as acetic acid ($CH_3COOH$), is the primary component of vinegar.
- Hydrochloric acid is incorrect: Hydrochloric acid ($HCl$) is a mineral acid found in gastric juice in the human stomach, where it aids in digestion.
- Sulphuric acid is incorrect: Sulphuric acid ($H_2SO_4$) is a strong mineral acid used extensively in industries (e.g., car batteries) and is not found in insect stings.
Key Takeaway:
Methanoic acid (Formic acid) is responsible for the pain in ant stings. The discomfort can be relieved by applying a mild base, such as baking soda, which neutralises the acid.