**Core Concept**
The potency of inhalational anesthetic agents is a critical factor in clinical anesthesia practice. It refers to the minimum alveolar concentration (MAC) of the anesthetic required to prevent movement in 50% of patients in response to a surgical stimulus, such as a painful stimulus.

**Why the Correct Answer is Right**
The correct answer is measured by the minimum alveolar concentration (MAC) of the anesthetic. MAC is a widely accepted standard for comparing the potency of inhalational anesthetics. It is defined as the concentration of anesthetic in the lungs that prevents movement in 50% of patients in response to a surgical stimulus. The MAC of an anesthetic is inversely related to its potency, meaning that a lower MAC value indicates a more potent anesthetic. The MAC values are typically expressed in percentages of the anesthetic's vapor pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the minimum alveolar concentration (MAC) is the correct measure of anesthetic potency, not the anesthetic's vapor pressure.
* **Option B:** This option is incorrect because the anesthetic's lipid solubility is related to its potency, but it is not the direct measure of potency.
* **Option C:** This option is incorrect because the anesthetic's blood/gas partition coefficient is related to its induction and recovery characteristics, but it is not the direct measure of potency.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the MAC of an anesthetic can be affected by various factors, such as age, obesity, and the presence of other medications, which can alter the anesthetic's potency.

**Correct Answer:** D. Minimum Alveolar Concentration (MAC)