Assertion: The resolving power of a telescope is more if the diameter of the objective lens is more.Reason: Objective lens of large diameter collects more light.

The resolving power of a telescope is given by $\frac{D}{1.22\lambda }$, where $D$ is the diameter of the objective lens. Increasing $D$ increases resolving power. While a larger lens does collect more light (improving brightness), the increase in resolving power is specifically due to the reduction in the diffraction limit. Both are true, but Reason is not the correct explanation for resolving power.