**Core Concept**
The management of thyrotoxicosis in pregnancy requires careful selection of medications to ensure the safety of both the mother and the fetus. The ideal agent should have a rapid onset of action, minimal side effects, and the ability to cross the placenta.

**Why the Correct Answer is Right**
Propranolol (C) is the agent of choice for treating thyrotoxicosis during pregnancy. It is a beta-blocker that helps control the symptoms of hyperthyroidism, such as palpitations and tremors, without causing significant fetal harm. Propranolol can be used throughout pregnancy, including the first trimester, and is effective in reducing the maternal heart rate and cardiac output.

**Why Each Wrong Option is Incorrect**
**Option A:** Methimazole is a thionamide antithyroid medication that can cause fetal abnormalities and is generally avoided in the first trimester of pregnancy.
**Option B:** Radioactive iodine is contraindicated in pregnancy due to the risk of thyroid ablation in the fetus.
**Option D:** Thionamides, such as propylthiouracil, are effective in treating hyperthyroidism but may cause hepatotoxicity and are often avoided in pregnancy.

**Clinical Pearl / High-Yield Fact**
Beta-blockers like propranolol can help alleviate symptoms of thyrotoxicosis in pregnancy, but they do not affect the underlying thyroid hormone levels.

**Correct Answer: C. Propranolol**