If the focal length of the objective lens is increased then

The correct option is D The magnifying power of microscope will decrease but that of the telescope will increase
A microscope consists of a lens of small focal lengths. A telescope consists of an objective lens of a large focal length.

For a microscope,

$m=\frac{L}{f_0}\times \frac{D}{f_e}$ and

FOr telescope,

$m=\frac{f_o}{f_e}$

Therefore, The magnifying power of microscope will decrease but that of the telescope will increase.