Magnifying power of simple microscope is 6, when image is formed at distance of distinct vision, the focal length of simple microscope is :

The correct option is C $5cm$
Given,

$m=6$ magnification of simple microscope

$D=25cm$ Least distance of distinct vision.

The magnification of simple microscope,

$m=1+\frac{D}{f}$

$6=1+\frac{25cm}{f}$

$5=\frac{25cm}{f}$

$f=\frac{25cm}{5}$

$f=5cm$

The correct option is C.