You should know¶
$$ \large M = \frac{f_o}{f_e} $$
where
$f_o$: focal length of the objective lense in a refractor or of the primary mirror in a reflector
$f_e$: focal length of the eyepiece
Solution¶
The separation between two lenses is 100 cm.
Since the eye-piece lense has a focal length of 20 cm, the focal length of the objectiec lense is 80 cm.
The angular magnification is
$$ \large M = \frac{f_o}{f_e} = \frac{80 \ \text{cm}}{20 \ \text{cm}} = 4 $$Answer¶
(A) 4