Electric Field & Field Lines question from MHT CET 2025 (26 Apr, Shift 2)

Answer: (a)

1. Opposite pairs $A, D$ ($+q, +q$) and $C, F$ ($-q, -q$) give zero field at $O$.
2. Field of the five charges = field of $-q$ at $B$: $\dfrac{kq}{r^2}$ towards $B$.
3. Field of $+Q$ at $E$: $\dfrac{kQ}{r^2}$, also towards $B$.
4. $\dfrac{kq}{r^2} = 2\dfrac{kQ}{r^2} \Rightarrow Q = \dfrac{q}{2}$
5. So the answer is (a) $\dfrac{q}{2}$.