d
(d) \(P = \frac{{{V^2}}}{R}\) \( \Rightarrow \frac{{{P_2}}}{{{P_1}}} = \frac{{V_2^2}}{{V_1^2}}\) ( is constant)
\( \Rightarrow \frac{{{P_2}}}{{{P_1}}} = {\left( {\frac{{100}}{{200}}} \right)^2} = \frac{1}{4}\)\( \Rightarrow {P_2} = \frac{{{P_1}}}{4} = \frac{{40}}{4} = 10\,W\)