Q.A telescope has an objective of diameter 2.5 m. What is its angular resolution when it observes at 5500 Å?

Resolving power of a telescope is governed by diffraction at its objective aperture, which is set by its diameter.

The resolving power of a telescope is its ability to distinguish two closely-spaced distant objects (e.g. two stars) as separate. It is limited by diffraction at the objective lens/mirror, and is given by the (minimum) angular separation that can just be resolved:

$d\theta=\frac{1.22\lambda}{a}$

where $a$ is the diameter (aperture) of the objective and $\lambda$ is the wavelength of light used.

Resolving power (the reciprocal of $d\theta$) is therefore

$\text{R.P.}=\frac{a}{1.22\lambda}$

…