**Core Concept**
The question is testing the knowledge of the half-lives of various anesthetic agents, specifically the longest half-life among them. The half-life of a drug refers to the time it takes for the plasma concentration of the drug to reduce by half, which is a crucial factor in determining the duration of action and the frequency of administration.

**Why the Correct Answer is Right**
The correct answer is **D. Thiopentone**. Thiopentone, also known as thiopental, is a barbiturate anesthetic agent with a long half-life compared to other anesthetic agents. Its half-life is approximately 10-22 hours, allowing for a longer duration of action and a less frequent need for redosing. This is due to its high lipid solubility, which allows it to be stored in fatty tissues and released slowly.

**Why Each Wrong Option is Incorrect**
**Option A:** Halothane is an inhalational anesthetic agent with a shorter half-life compared to thiopentone. Its half-life is approximately 2-10 hours, making it less suitable for prolonged surgical procedures.

**Option B:** Ketamine is an anesthetic agent with a short half-life, typically ranging from 2-3 hours. Its short duration of action requires more frequent dosing, making it less ideal for prolonged surgical procedures.

**Option C:** Propofol is an anesthetic agent with a short half-life, typically ranging from 1-4 hours. Its short duration of action requires more frequent dosing, making it less suitable for prolonged surgical procedures.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the choice of anesthetic agent depends on the duration of surgery, the patient's medical history, and the desired level of sedation. Thiopentone's long half-life makes it suitable for short to moderate-length surgeries, while agents like propofol and ketamine are better suited for shorter procedures.

**Correct Answer: D. Thiopentone**