**Core Concept**
During pregnancy, the use of certain medications is carefully evaluated due to the potential risks to the fetus. The primary concern is to minimize harm while treating the mother's condition effectively. The safety of drugs in pregnancy is typically classified into three categories: A (safe), B (caution advised), and C (potential risks outweigh benefits).

**Why the Correct Answer is Right**
The correct answer is a drug that has been extensively studied and found to be safe for use in pregnancy, with a favorable benefit-to-risk ratio. This drug is often used to treat a common condition in pregnant women, such as hyperemesis gravidarum or pre-eclampsia. The mechanism of action involves the inhibition of serotonin reuptake, which helps to reduce nausea and vomiting.

**Why Each Wrong Option is Incorrect**
**Option A:** This drug is a known teratogen and is contraindicated during pregnancy due to its potential to cause fetal malformations.
**Option B:** Although this drug is used to treat a common condition in pregnancy, it is not recommended due to its potential to cause fetal hypotension and bradycardia.
**Option C:** This drug is a known cause of fetal cardiac malformations and is therefore contraindicated during pregnancy.

**Clinical Pearl / High-Yield Fact**
When prescribing medications during pregnancy, it's essential to consider the potential risks and benefits to both the mother and the fetus. Always consult the latest guidelines and consult with a specialist if necessary.

**Correct Answer:** D. Ondansetron.