What is the reduced form of cysteine called?

The reduced form of cysteine is indeed cysteine itself. In biochemical terms, cysteine is an amino acid that contains a thiol (-SH) group, which allows it to act in redox reactions. When in its reduced form, cysteine has a free thiol group.

In contrast, when two cysteine molecules undergo oxidation, they can form a covalent bond to create cystine, which is the oxidized form consisting of two cysteine residues linked by a disulfide bond. The term 'oxidized cysteine' would typically refer to this same concept. The option 'redox cysteine' is not a standard term used in biochemistry and does not accurately describe either the reduced or oxidized states of the amino acid. Therefore, the only accurate representation of the reduced form is simply cysteine. This distinction is crucial in understanding how cysteine functions within proteins, especially in forming and breaking disulfide bonds, which contribute to the protein's structure and function.