What concept is considered when generic drugs are substituted for brand-name drugs?

A. Bioavailability refers to the proportion of a drug that enters systemic circulation and is available for therapeutic effect. Generic drugs must demonstrate bioequivalence to brand-name drugs, meaning their bioavailability must be within an acceptable range to ensure similar efficacy. B. Critical concentration is the minimum drug level required to produce a therapeutic effect, but it is not the primary factor in generic drug substitution. C. Distribution refers to how a drug is transported in the body after absorption, but it is not the main concern when substituting generic drugs. D. Half-life is the time required for the drug concentration to decrease by half in the body, but it does not determine whether a generic drug can replace a brand-name drug. Bioavailability is the key factor in ensuring therapeutic equivalence.