**Core Concept**
100% bioavailability refers to the administration of a drug in a form that is immediately available for absorption by the body without any first-pass metabolism or degradation. This concept is crucial in pharmacokinetics, as it directly affects the drug's efficacy and dosage.

**Why the Correct Answer is Right**
Sublingual administration involves placing a drug under the tongue, where it is absorbed directly into the bloodstream through the oral mucosa. Since this route bypasses the gastrointestinal tract and liver, the drug is not subjected to first-pass metabolism, ensuring that 100% of the administered dose is available for therapeutic action. The rapid absorption and high bioavailability of sublingual administration make it a preferred route for certain medications, such as nitroglycerin and fentanyl.

**Why Each Wrong Option is Incorrect**
**Option A:** Intravenous (IV) administration is known for its high bioavailability, but it is not 100% due to potential losses during preparation and administration. Additionally, IV administration bypasses the gastrointestinal tract but still involves first-pass metabolism in the liver.
**Option B:** Oral administration involves first-pass metabolism in the liver, which reduces bioavailability. While certain oral medications have high bioavailability, it is not 100% due to this factor.
**Option C:** Topical administration, such as transdermal patches, has varying bioavailability depending on the medication and skin absorption rates. While some topical medications have high bioavailability, it is not 100% due to skin metabolism and other factors.

**Clinical Pearl / High-Yield Fact**
Sublingual administration is a high-yield fact for exams, as it is a unique route that bypasses first-pass metabolism, ensuring high bioavailability. Remember that sublingual administration is often used for medications that require rapid action, such as cardiovascular emergencies.

**Correct Answer: D. Sublingual administration.**