**Core Concept:** Bioavailability is a pharmacokinetic parameter that represents the fraction of an orally administered drug that reaches the systemic circulation. In this case, we are given the absorption percentage and hepatic extraction ratio (HER) to calculate bioavailability.

**Why the Correct Answer is Right:** To calculate bioavailability (F), we use the following formula:

F = (AUC_oral / AUC_intramuscular) x 100%

Here, AUC_oral and AUC_intramuscular represent the area under the concentration-time curve for oral and intramuscular administration, respectively. Since the drug has 80% absorption, AUC_oral is 80% of the AUC_intramuscular.

F = (0.8 / 1) x 100% = 80%

So, the bioavailability is 80%.

**Why Each Wrong Option is Incorrect:**

A. Bioavailability cannot be determined without knowing the absorption percentage or AUC values.
B. Bioavailability is not calculated using HEPATIC extraction ratio.
C. Bioavailability is not calculated using both absorption percentage and HEPATIC extraction ratio simultaneously; these are separate parameters.
D. Bioavailability is not calculated using AUC values from different administration routes (oral and intramuscular).

**Clinical Pearl:** Bioavailability is crucial to understand when assessing drug efficacy, selecting the appropriate dosage form, and predicting drug interactions. A low bioavailability may indicate poor absorption or extensive first-pass liver elimination, leading to reduced therapeutic effect and potential need for dosage adjustment.

**Correct Answer:**

B. Bioavailability = (AUC_oral / AUC_intramuscular) x 100%

By understanding the concept of bioavailability, pharmacologists and physicians can optimize drug administration, ensuring patients receive the desired therapeutic effect without unnecessary side effects or toxicities.