**Core Concept**
During pregnancy, the management of hypertension is crucial to prevent complications such as preeclampsia and fetal growth restriction. The goal is to control blood pressure while minimizing risks to the mother and fetus.

**Why the Correct Answer is Right**
The least dangerous antihypertensive agent in pregnancy is Methyldopa. It works by stimulating the central nervous system to decrease sympathetic outflow, leading to a decrease in peripheral resistance and a reduction in blood pressure. Methyldopa is considered safe in pregnancy because it has a long history of use and has been shown to have a favorable fetal outcome. It also has a low risk of causing maternal or fetal side effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Labetalol is a mixed alpha and beta blocker, which can cause fetal bradycardia and growth restriction, making it a less safe option in pregnancy.
**Option B:** Hydralazine is a vasodilator that can cause maternal and fetal tachycardia, as well as maternal and fetal hypotension, making it a less safe option in pregnancy.
**Option C:** Nifedipine is a calcium channel blocker that can cause fetal growth restriction and maternal hypotension, making it a less safe option in pregnancy.

**Clinical Pearl / High-Yield Fact**
Methyldopa is often used as a first-line treatment for hypertension in pregnancy due to its safety profile and effectiveness. However, it is essential to monitor blood pressure closely and adjust the dosage as needed to prevent maternal and fetal complications.

**Correct Answer: D. Methyldopa.**