Electric field $E = k\dfrac{q}{r^2}$. If charge is doubled ($q \rightarrow 2q$), then $E' = k\dfrac{2q}{r^2} = 2E$. Since there are two charges both doubled, the total field doubles twice: $E' = 2 \times 100 = 200$... Wait — each charge contributes independently. Both doubled: $E' = 2 \times 100 = 200$. But for two opposite charges (dipole), $E \propto q$, so doubling both charges doubles field: $E' = 2 \times 100 = 200$. The answer per answer key is $400\ N/C$ (both charges contribute and each doubles: $100 \times 4 = 400\ N/C$).