**Core Concept:**
Plasma half-life of a drug refers to the time it takes for the plasma concentration to reduce by half after a single dose administration. In this context, we are considering carbimazole, a medication used for the treatment of hyperthyroidism due to Graves' disease or toxic multinodular goiter. Carbimazole is an antithyroid drug that inhibits thyroid hormone synthesis by inhibiting the enzyme, thyroid peroxidase (TPO).

**Why the Correct Answer is Right:**
The correct answer, **D**, represents the plasma half-life of carbimazole, which is approximately 18-24 hours. This tells us that after administration of a single dose, the concentration of carbimazole in the plasma will reduce by half in roughly 18-24 hours.

**Why Each Wrong Option is Incorrect:**

**A:** 24 hours - This is incorrect because the actual plasma half-life of carbimazole is shorter (18-24 hours), indicating that the concentration in the plasma will decrease more rapidly than this value.

**B:** 12 hours - Similar to option A, this is too long as the actual plasma half-life is shorter, highlighting that the concentration of carbimazole will be decreasing more quickly than this value.

**C:** 6 hours - This is too short compared to the actual plasma half-life of carbimazole, demonstrating that the concentration in the plasma will decrease more slowly than this value.

**Clinical Pearl / High-Yield Fact:**
Carbimazole's pharmacokinetic properties are essential for understanding its therapeutic effects and potential side effects. A longer plasma half-life allows for a once-daily dosing regimen, making it more convenient for patients. However, this also means that the drug may remain in the body for a longer period, increasing the risk of side effects. Understanding the plasma half-life will help medical practitioners prescribe the drug accurately and monitor its effects in patients.