The magnifying power of a compound microscope increases when

The correct option is D

Focal length of both objective and eyepiece are decreased.

Explanation for the correct option

Option (D)

1. Compound microscopes are used to see very small objects (In the range of nm).
2. The magnification power of a compound microscope is given by the form, $m=-\frac{1}{f_o}\frac{D}{f_e}$, where, $f_o$ is the focal length of the objective, $f_e$ is the focal length of the eyepiece, and $D$ is the least distance of clear vision.
3. It is clear that if we decrease the focal length of the objective $\left(f_o\right)$ and eyepiece $\left(f_e\right)$, then the magnification of the microscope will be increased.

Diagram

So, option (D) is correct.