A

The Cauchy’s dispersion formula describes how the refractive index of a material depends on the wavelength of light. The correct expression typically includes terms representing the dispersion coefficients and wavelengths, which can be represented in the formula shown in Option A.
Step 1: Identification of Dispersion Formula - Cauchy's equation is generally represented in the form:
$$ n(\lambda) = A + \frac{B}{\lambda^2} + \frac{C}{\lambda^4} + ... $$
where \( n \) is the refractive index, \( \lambda \) is the wavelength, and A, B, and C are constants for the material.
Step 2: Examination of Options - Upon reviewing the options, Option A aligns with the standard form of Cauchy's dispersion formula, indicating that it is the correct choice.
Therefore, the answer is A.