Answer:
E) d/sqrt2
Step-by-step explanation:
The initial electric force between the two charge is given by:

where
k is the Coulomb's constant
q1, q2 are the two charges
d is the separation between the two charges
We can also rewrite it as

So if we want to make the force F twice as strong,
F' = 2F
the new distance between the charges would be

so the correct option is E.