To increase magnification power of refracting type Telescope, we should increase:

the focal length of the objective

The correct answer is Option (1) → the focal length of the objective

The magnifying power (M) of a refracting telescope is,

$M=\frac{f_o}{f_e}$  [$f_0$ = focal length of objective]

Hence, by increasing the focal length of the objective lens, one increases magnification.