Resolving power of a microscope can be increased, if

The correct option is D Both $\left(\text{A}\right)$ and $\left(\text{C}\right)$
The resolving power of a microscope is,

$RP=\frac{2\mu \sin \theta }{1.22\lambda }$

Where,

$\mu =\text{Refractive index of medium between object and objective lens}\theta =\text{Angular resolution (in radians)}\lambda =\text{Wavelength of the light}$

$\Rightarrow RP\propto \mu \text{and}RP\propto \frac{1}{\lambda }$

Since, we know that red light has higher wavelength as compare to blue. Thus, blue light will increase the value of $RP$.

Since value of $\mu$ increases due to oil, so $RP$ will increase.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(\text{D}\right)$ is the correct answer.

Why this question? Key concept : Resolving power of a microscope is inversely proportional to the wavelength of light used.