If a charge q is placed at the centre of the line joining two equal charges Q such that the system is in equilibrium then the value of q is
F = q1q2/4πεr2
For equilibrium charges, the net force should be zero.
Select a position of charge q on the line joining Q1 and Q2 and use formula of force and equate the sum of forces to zero to find the value of q.
The correct option is B.