A compound microscope has objective focal length $f_o$ and eyepiece focal length $f_e$. If all other parameters remain unchanged, which change will increase its magnifying power?

Using $m\approx (L/f_o)\times(D/f_e)$, the magnifying power increases when $L$ increases, $f_o$ decreases, or $f_e$ decreases. Therefore decreasing both focal lengths increases magnifying power. Option D is wrong because reducing $L$ reduces magnification.