Answer:
b. Covalent bond between R groups
Explanation:
Disulfide bridge also known as disulfide bond or dicysteine bond is a type of interaction in which two cysteines of proteins come in close proximity with each other and form covalent bond with their functional (R) groups. The purpose of this bond is to stabilize the tertiary structure of a protein and make it more compact.
Please see the attached image for more understanding.
