Two charged particles, placed at a distance d apart in vacuum, exert a force F on each other. Now, each of the charges is doubled. To keep the force unchanged, the distance between the charges should be changed to ________.

Fill in the blank with the correct answer from the options given below.

$2 d$

$ F = \frac{kq_1q_2}{d^2} = \frac{k2q_1 \times  2q_2}{d'^2}$

$\Rightarrow d' = 2d$

The correct answer is Option (2) → $2 d$