In a microscope of tube length 10 cm two convex lenses are arranged...

For a microscope in normal adjustment, magnification is given by $M = \frac{L}{f_o} \times \frac{D}{f_e}$
where $L$ is tube length (10 cm), $f_o$ is objective focal length (2 cm), $f_e$ is eyepiece focal length (5 cm), and $D$ is least distance of distinct vision (25 cm).
$M = \frac{10}{2} \times \frac{25}{5} = 5 \times 5 = 25 = 5^2$
Therefore, $k = 2$