Đáp án:

 D

Giải thích các bước giải:

Khi mắc nối tiếp: 

\(Q = \dfrac{{{U^2}}}{{{R_1} + {R_2}}}.{t_1} = \dfrac{{{U^2}}}{{2R}}.{t_1}\)

Khi mắc song song:

\(\begin{array}{l}
Q = \dfrac{{{U^2}}}{{\dfrac{{{R_1}{R_2}}}{{{R_1} + {R_2}}}}}.{t_2} = \dfrac{{{U^2}}}{{\dfrac{R}{2}}}.{t_2} = \dfrac{{2{U^2}}}{R}.{t_2}\\
 \Rightarrow \dfrac{{{U^2}}}{{2R}}.{t_1} = \dfrac{{2{U^2}}}{R}.{t_2}\\
 \Rightarrow {t_2} = \dfrac{{{t_1}}}{4} = \dfrac{{15}}{4} = 3,75ph
\end{array}\)