The correct answer is (b), sulfenic acid.
Onions contain a sulfur-containing compound called alliin. When an onion is cut or crushed, an enzyme called allinase converts alliin into allicin, which is responsible for the onion’s characteristic odor. Allicin is also a powerful irritant that can cause the eyes to tear.
The sulfur in allicin reacts with water to form sulfenic acid, which is the compound that actually causes the tears. Sulfenic acid is a strong acid that can irritate the eyes and cause them to water.
The other options are incorrect. Sulphonic acid is not a compound that is found in onions. Amino acids are the building blocks of proteins, and they are not known to cause tears. Carbolic acid is a type of phenol, and it is not known to cause tears either.
In addition to sulfenic acid, onions also contain other compounds that can irritate the eyes, such as syn-propanethial-S-oxide. These compounds can cause the eyes to water, burn, and itch.
If you are sensitive to onions, you can take steps to reduce the amount of irritation they cause. For example, you can wear goggles when you cut onions, and you can cook them in a well-ventilated area.