**Core Concept**
Therapeutic drug monitoring (TDM) is a clinical practice of measuring the concentration of drugs in the bloodstream to optimize their therapeutic effects while minimizing the risk of toxicity. This is particularly crucial for drugs with narrow therapeutic indexes, where small variations in plasma concentrations can significantly affect the efficacy and safety of the treatment.

**Why the Correct Answer is Right**
The correct answer is **Phenobarbital**. Phenobarbital is a barbiturate with a narrow therapeutic index, meaning that the difference between the effective dose and the toxic dose is small. TDM is essential for phenobarbital to prevent toxicity, which can manifest as sedation, confusion, and in severe cases, respiratory depression. The therapeutic range for phenobarbital is generally considered to be between 15-40 mg/L, and levels above this range can increase the risk of toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Digoxin - While TDM is useful for digoxin, it is not as critical as it is for phenobarbital. Digoxin has a wider therapeutic index, and toxicity is more often related to factors such as renal impairment and hypokalemia.
* **Option B:** Phenytoin - Phenytoin is another drug that requires TDM due to its narrow therapeutic index. However, phenobarbital is a more specific example of a drug that requires TDM due to its high risk of toxicity at therapeutic doses.
* **Option C:** Metformin - Metformin is a medication used to treat type 2 diabetes, and TDM is not typically required for this drug. Metformin is generally well-tolerated, and its efficacy and safety are not as closely tied to plasma concentrations as they are for drugs like phenobarbital.

**Clinical Pearl / High-Yield Fact**
When ordering TDM for a patient on phenobarbital, it's essential to consider the patient's individual factors, such as age, weight, and liver function, as these can affect the drug's clearance and plasma concentrations. Additionally, TDM should be performed at steady-state, typically after 5-7 half-lives of the drug, to ensure accurate measurements.

**Correct Answer:** C.