A simple microscope has a magnifying power of $3.0$ when the image is formed at the near point of a normal eye. What is its focal length?

The correct option is C $12.5\text{cm}$
Given:

Magnifying power, $m=3$

We know, when the image is formed at near point of a normal eye, the magnifying power is,

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

$\Rightarrow 3=1+\frac{25}{f}$

$\Rightarrow f=12.5\text{cm}$

Hence, option $\left(C\right)$ is the correct answer.