Explanation:

Propranolol undergoes extensive first-pass hepatic metabolism in the liver. When administered orally, propranolol is extensively metabolized by the liver before reaching systemic circulation, leading to reduced bioavailability. This process is known as first-pass hepatic metabolism, which significantly affects the drug's effectiveness and necessitates higher oral doses compared to other routes of administration. Heparin (Choice A) is not metabolized by the liver but excreted unchanged by the kidneys. Insulin (Choice B) is a peptide hormone that is not subject to significant first-pass metabolism. Nitroglycerin (Choice D) is primarily metabolized in the blood and tissues, bypassing significant first-pass metabolism in the liver.