**Core Concept**
The placental barrier is a selective permeability barrier that separates the mother's bloodstream from the fetus's bloodstream. It allows certain substances to pass through while preventing others, primarily based on molecular size and lipid solubility.

**Why the Correct Answer is Right**
The placental barrier is more permeable to lipid-soluble substances and smaller molecules. However, certain drugs, particularly those with a high molecular weight or those that are highly protein-bound, are unable to cross the placental barrier. One such example is heparin, which is a highly charged, high molecular weight anticoagulant that does not cross the placental barrier.

**Why Each Wrong Option is Incorrect**
**Option A:** **Incorrect** because it is not specified. However, if we consider a typical example of a drug that crosses the placental barrier, such as aspirin (acetylsalicylic acid), it would be incorrect. Aspirin is a small, lipid-soluble molecule that can cross the placental barrier.
**Option B:** **Incorrect** because it is also not specified. However, if we consider a typical example of a drug that crosses the placental barrier, such as digoxin, it would be incorrect. Digoxin is a cardiac glycoside that can cross the placental barrier.
**Option C:** **Incorrect** because it is also not specified. However, if we consider a typical example of a drug that crosses the placental barrier, such as phenytoin, it would be incorrect. Phenytoin is an antiepileptic medication that is lipid-soluble and can cross the placental barrier.

**Clinical Pearl / High-Yield Fact**
It is essential to note that while heparin does not cross the placental barrier, it can cause osteoporosis in the fetus due to the inhibition of bone growth. This highlights the importance of choosing anticoagulants wisely during pregnancy.

**Correct Answer:** . Heparin.