Thiol group is present in:

A. cystine

B. cysteine

C. methionine

D. cytosine

A thiol group (-SH) is a sulfur-containing functional group. Among the options:

• Cysteine contains a free thiol (-SH) group.
• Cystine is formed when two cysteine molecules are linked via a disulfide bond (-S-S-), so the free thiol group is not present.
• Methionine contains sulfur, but as a thioether (-S-), not as a thiol.
• Cytosine is a nitrogenous base and does not contain sulfur.

So, cysteine is the amino acid with a free thiol group.

The correct answer is: B. cysteine