**Core Concept**
Heparin is an anticoagulant medication that works by enhancing the activity of **antithrombin**, a naturally occurring inhibitor of thrombin and other clotting factors. It is commonly used to prevent and treat thrombosis. Heparin's mechanism of action involves binding to **antithrombin**, increasing its affinity for **thrombin** and **factor Xa**.

**Why the Correct Answer is Right**
Since the options are missing, let's discuss the general properties of heparin. Heparin is administered parenterally, either intravenously or subcutaneously, due to its poor oral bioavailability. It has a rapid onset of action and is often monitored using **activated partial thromboplastin time (aPTT)**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, it's challenging to provide a detailed explanation. However, if an option stated that heparin is administered orally, it would be incorrect due to heparin's poor oral bioavailability.
**Option B:** Similarly, without the option, we can't provide a specific reason. But if it claimed heparin has a delayed onset of action, that would be incorrect as heparin acts rapidly.
**Option C:** And **Option D:** would be incorrect based on their specific claims about heparin, which could include incorrect statements about its mechanism, monitoring, or administration.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that heparin's effect can be reversed with **protamine sulfate**, which is especially important in situations where rapid reversal of anticoagulation is necessary, such as after cardiac surgery or in cases of heparin overdose.

**Correct Answer:** D. Heparin is administered orally.