**Core Concept**
Thyroid storm, a life-threatening complication of untreated or inadequately treated hyperthyroidism, is a concern in pregnant women with Graves' disease. The goal of treatment is to control hyperthyroidism and prevent thyroid storm while protecting the fetus.

**Why the Correct Answer is Right**
The correct answer is **C. Methimazole**. Methimazole, a thionamide antithyroid drug, is the preferred initial treatment for hyperthyroidism in pregnancy due to its effectiveness and relatively lower risk of causing congenital anomalies compared to other options. Methimazole works by inhibiting thyroid hormone synthesis, reducing the production of triiodothyronine (T3) and thyroxine (T4).

**Why Each Wrong Option is Incorrect**
**Option A:** Radioactive iodine is contraindicated in pregnancy due to the risk of fetal thyroid ablation and subsequent cretinism.
**Option B:** Propranolol, a beta-blocker, may be used to control symptoms of hyperthyroidism such as tremors and tachycardia, but it does not treat the underlying cause of hyperthyroidism.

**Clinical Pearl / High-Yield Fact**
Methimazole is the preferred antithyroid drug in pregnancy, but it requires careful monitoring of TSH and thyroid hormone levels to avoid potential side effects.

**Correct Answer: C. Methimazole**