A

The resolving power of a microscope defines its ability to distinguish between two closely spaced objects. When dealing with a self-luminous object, the expression for resolving power is typically derived from the diffraction limit of the lens used in microscopy.
Step 1: The resolving power (R) is given by the formula:
$$ R = \frac{1.22 \lambda}{D} $$
where \(\lambda\) is the wavelength of light used and \(D\) is the diameter of the aperture of the lens. This formula arises from considering the diffraction pattern created by the optical system.
Step 2: Since the options provide visual representations, we can identify that the expression in option A aligns with this equation.
Therefore, the correct answer is A.