**Core Concept**
Thiopentone is a barbiturate used in the induction of general anesthesia. It acts by depressing the activity of the central nervous system (CNS) through the enhancement of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA).

**Why the Correct Answer is Right**
Thiopentone is a highly lipid-soluble agent, which allows it to quickly cross the blood-brain barrier and produce rapid onset of action. The recommended dose of thiopentone for induction of anesthesia is typically 4-7 mg/kg, and it is usually administered at a concentration of 2.5% (25 mg/mL). The correct percentage of thiopentone used in induction is 2.5%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a commonly used concentration of thiopentone for induction.
* **Option B:** This option is incorrect because it is a higher concentration than typically used for induction.
* **Option C:** This option is incorrect because it is a lower concentration than typically used for induction.

**Clinical Pearl / High-Yield Fact**
When administering thiopentone, it is essential to ensure that the patient's airway is protected, as thiopentone can cause respiratory depression.

**Correct Answer: D. 2.5%**