\(E_{AC}=\frac{k.q_1}{r^2}=\frac{9.10^9.36.10^{-7}}{0,3^2}=360000\left(V/m\right)\)
\(E_{BC}=\frac{kq_2}{r'^2}=\frac{9.10^9.36.10^{-7}}{0,4^2}=202500\left(V/m\right)\)
\(\Rightarrow E=\sqrt{E_{AC}^2+E_{BC}^2}==\sqrt{360000^2+202500^2}=41,3.10^4\left(V/m\right)\Rightarrow D\)