**Core Concept:** The encircled number in the instrument formula represents the number of elements in a specific type of instrument. In this case, it is for a compound microscope. The formula for compound microscopes is 10-(80)-6-12, indicating the magnification powers of the objective lenses, eyepiece, field of view, and eye relief respectively.

**Why the Correct Answer is Right:**

The correct answer is **D**. In a compound microscope, the objective lenses are arranged in a series, with each lens having a lower magnification power than the previous one. The encircled number represents the magnification power of the eyepiece lens, which is 12x in this case.

**Why Each Wrong Option is Incorrect:**

Option A (80) is incorrect because it represents the magnification power of the first objective lens, which is 80x in this case.

Option B (6) is incorrect because it represents the field of view, which is determined by the combined magnification powers of the objective lenses and eyepiece.

Option C (10) is incorrect because it represents the magnification power of the second objective lens, which is 10x in this case.

**Why the Correct Answer is Right:**

In a compound microscope, the eyepiece lens is the highest magnification component, providing the final magnification for the user. In this case, the eyepiece lens has a magnification power of 12x, resulting in a final magnification of 1200x when combined with the 10x and 20x objective lenses.

**Clinical Pearl:**

A compound microscope is a fundamental tool in medical practice, allowing for detailed examination of microorganisms, cells, and tissues. Understanding the instrument formula helps students and professionals correctly set up the microscope for optimal visualization and accurately interpret the observed specimens.