Lời giải chi tiết:

Ta có:

\(\begin{array}{l}
E = \frac{{k.\left| Q \right|}}{{{r^2}}} \Rightarrow \left\{ \begin{array}{l}
{E_1} = \frac{{k.\left| Q \right|}}{{r_1^2}} = {10^5}\\
{E_2} = \frac{{k.\left| Q \right|}}{{r_2^2}} = {4.10^5}
\end{array} \right.\\
\Rightarrow \frac{{{E_1}}}{{{E_2}}} = \frac{{r_2^2}}{{r_1^2}} = \frac{1}{4} \Rightarrow {r_2} = \frac{{{r_1}}}{2} = \frac{2}{2} = 1cm
\end{array}\)

Chọn B.