**Core Concept**
The therapeutic index (TI) of a drug is a quantitative measure of its safety and efficacy, calculated as the ratio of the dose that produces a therapeutic effect to the dose that produces toxicity. It is a crucial pharmacological parameter that helps in assessing the risk-benefit ratio of a medication.

**Why the Correct Answer is Right**
The TI is indicative of a drug's margin of safety, which is the difference between the dose required to produce a therapeutic effect and the dose that causes adverse effects. A higher TI indicates a wider margin of safety, whereas a lower TI suggests a narrower margin of safety. The TI takes into account the dose-response curves for both therapeutic and toxic effects of a drug, providing a useful tool for clinicians to weigh the benefits against the risks of treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the TI does not directly relate to the drug's mechanism of action or its pharmacokinetic properties, such as bioavailability or half-life.
**Option B:** This option is incorrect because the TI is not a measure of a drug's efficacy or potency, although it is related to the dose required to produce a therapeutic effect.
**Option C:** This option is incorrect because the TI does not specifically indicate the drug's duration of action or its peak plasma concentration.

**Clinical Pearl / High-Yield Fact**
A higher TI is generally desirable, as it indicates a safer and more effective medication. However, it's essential to note that some drugs with a narrow TI may still be used in clinical practice, particularly if they offer significant benefits that outweigh the risks.

**Correct Answer: A. Therapeutic index of a drug is indicative of its margin of safety.**