**Core Concept**
The therapeutic index (TI) of a drug is a measure of its safety, calculated as the ratio of the dose that produces a toxic effect to the dose that produces the desired therapeutic effect. It is an indicator of the margin between the effective dose and the toxic dose of a drug.

**Why the Correct Answer is Right**
To calculate the therapeutic index, we need to identify the dose that produces the desired therapeutic effect (ED50) and the dose that produces a toxic effect (TD50). From the graph, we can see that the ED50 is approximately 100 mg/kg, while the TD50 is approximately 200 mg/kg. Therefore, the therapeutic index (TI) is calculated as TD50/ED50 = 200/100 = 2. This indicates that the drug is relatively safe, with a two-fold margin between the effective dose and the toxic dose.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but we can assume that the correct answer is not listed here.
* **Option B:** This option is also not provided, but we can assume that the correct answer is not listed here.
* **Option C:** This option is not provided, but we can assume that the correct answer is not listed here.
* **Option D:** This option is also not provided, but we can assume that the correct answer is not listed here.

**Clinical Pearl / High-Yield Fact**
When interpreting the therapeutic index, a higher value indicates a safer drug, while a lower value indicates a more toxic drug. It's essential to consider the therapeutic index when selecting a medication, as it can help minimize the risk of adverse effects.

**Correct Answer:** .