**Core Concept**
Multiple carboxylase deficiency (MCD) is a rare genetic disorder caused by a deficiency of one of the two enzymes, holocarboxylase synthetase (HCS) or biotinidase, which are essential for the metabolism of biotin. Biotin is a coenzyme required for various carboxylation reactions in the body, including the metabolism of fatty acids, amino acids, and glucose.

**Why the Correct Answer is Right**
The correct treatment for MCD involves the administration of biotin supplements, which can help bypass the defective enzyme and restore normal biotin metabolism. Biotin supplements can be given orally or intravenously, depending on the severity of the deficiency and the patient's clinical condition. The recommended dose of biotin is typically 10-20 mg/day, but this may vary depending on the individual case.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because vitamin B12 supplements are not effective in treating multiple carboxylase deficiency. While vitamin B12 is also a coenzyme involved in carboxylation reactions, its metabolism is not directly affected by the enzymes deficient in MCD.

**Option B:** This option is incorrect because folic acid supplements are not effective in treating multiple carboxylase deficiency. Folic acid is a different coenzyme involved in various metabolic reactions, but it is not directly related to the enzymes deficient in MCD.

**Option C:** This option is incorrect because carnitine supplements are not effective in treating multiple carboxylase deficiency. Carnitine is an amino acid involved in the transport of fatty acids into mitochondria, but its metabolism is not directly affected by the enzymes deficient in MCD.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that multiple carboxylase deficiency can present with a wide range of clinical symptoms, including skin rash, alopecia, and developmental delay. Early diagnosis and treatment with biotin supplements can help prevent long-term complications and improve the patient's quality of life.

**Correct Answer: D. Biotin supplements.**