The correct answer is option 1: sulfur in the cell.

When cutting or peeling onions, a substance called alliinase is released. Alliinase is an enzyme that reacts with a sulfur-containing compound called cysteine sulfoxide, which is present in the cells of the onion. This reaction produces a volatile gas called syn-propanethial-S-oxide.

When this gas comes in contact with the nerve endings in the eyes, it forms a mild sulfuric acid, which irritates the eyes and stimulates the tear glands, leading to the production of tears. This is why cutting onions can make our eyes tear up.

The other options, carbon, fat, and amino acids, are not responsible for the tear-inducing effect of onions. While carbon is an essential element in living organisms, it does not directly cause tearing. Fat and amino acids are important components of cells, but they do not have the same chemical composition that leads to eye irritation and tear production.

Therefore, option 1 - sulfur in the cell - is the correct answer.