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

-Q/4

The correct answer is option 4: -Q/4

Since all the charges are in equilibrium then force on charge Q due to other charges are zero

$\Rightarrow \frac{kqQ}{d^2} + \frac{kQ.Q}{4d^2} = 0$

$\Rightarrow q = \frac{-Q}{4}$