**Core Concept**
The question is testing the student's knowledge of the mechanism of action of antithyroid drugs, specifically their effects on thyroid hormone synthesis. Iodine trapping is the initial step in thyroid hormone synthesis, where iodide ions are transported into the thyroid gland.

**Why the Correct Answer is Right**
The correct answer, **Propylthiouracil (PTU)**, is a prodrug that inhibits the enzyme thyroid peroxidase (TPO), which is necessary for iodine oxidation and coupling of iodotyrosines. However, it also has a secondary effect of inhibiting the sodium-iodide symporter (NIS), which is responsible for iodide uptake (iodine trapping) into the thyroid gland. This is why PTU is effective in reducing iodine trapping.

**Why Each Wrong Option is Incorrect**
* **Option A: Methimazole** is a more potent inhibitor of thyroid peroxidase, but it does not significantly affect iodine trapping.
* **Option B: Iodide** actually increases iodine trapping by competitively inhibiting the sodium-iodide symporter, so it is not an antithyroid drug that inhibits iodine trapping.
* **Option C: Thiamazole** (another name for Methimazole) has a similar mechanism of action to Methimazole, with no significant effect on iodine trapping.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that PTU has a unique dual mechanism of action, making it a valuable option in treating hyperthyroidism, particularly in patients with iodine-induced thyrotoxicosis.

**Correct Answer: C. Iodide**