**Core Concept**
The metabolism of sulphur-containing amino acids, such as methionine and cysteine, involves a series of biochemical reactions that require the assistance of cofactors. These cofactors play a crucial role in facilitating the transfer of sulphur groups and maintaining the balance of sulphur-containing compounds in the body.

**Why the Correct Answer is Right**
The correct answer is **Biotin**. Biotin is a crucial cofactor in the metabolism of sulphur-containing amino acids, particularly in the transsulphuration pathway. This pathway involves the conversion of methionine to cysteine, which is essential for the synthesis of glutathione, a key antioxidant in the body. Biotin acts as a coenzyme for the enzyme cystathionine beta-synthase, which catalyzes the condensation of homocysteine and serine to form cystathionine, a crucial step in the transsulphuration pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **Thiamine** is a cofactor involved in carbohydrate metabolism, particularly in the decarboxylation of alpha-keto acids in the Krebs cycle. It does not play a direct role in sulphur-containing amino acid metabolism.

**Option C:** This option is incorrect because **Riboflavin** is a cofactor involved in energy metabolism, particularly in the transfer of electrons in the electron transport chain. It does not have a direct role in sulphur-containing amino acid metabolism.

**Clinical Pearl / High-Yield Fact**
Biotin deficiency can lead to a range of clinical symptoms, including dermatitis, alopecia, and neurological disorders. It is essential to recognize biotin deficiency in patients with malabsorption syndromes or those taking anticonvulsant medications, which can interfere with biotin absorption.

**Correct Answer:** B. Biotin