You can tell if vector is an unit because it has something to do with taking the magnitude of the vector.
also has a length 1. thus you should use pythagoras to check.
Hope this help
Answer:
When one of charge is doubled, the magnitude of the force between them gets doubled.
Explanation:
The electric force between two electric charges is given by :

Here,
k is the electrostatic force
d is the separation between charges
are charges
If one of the charges is doubled in magnitude while maintaining the same separation between the charges, 
New force becomes,



When one of charge is doubled, the magnitude of the force between them gets doubled. Hence, this is the required solution.