Resolving power of a telescope can be increased by increasing

The resolving power of a telescope is determined by its ability to distinguish between closely spaced objects and is given by the formula $RP = \frac{D}{1.22\lambda}$, where $D$ is the diameter of the objective lens and $\lambda$ is the wavelength of light.

Since resolving power is directly proportional to $D$ and inversely proportional to $\lambda$, increasing the objective lens diameter enhances resolution, while increasing the wavelength diminishes it. The eyepiece diameter and focal length affect magnification but not resolving power.

Thus, increasing the diameter of the objective lens improves the telescope's resolving power.

Answer: D