Which of the following undergo nitration at fastest rate?\
(A) $\left. \mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { NO } _ { 2 } \right) ^ { - R }$\
(B) $\mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { CH } _ { 3 }$\
(C) $\mathrm { C } _ { 6 } \mathrm { H } _ { 3 } \underset { - R } { \mathrm { COOH } }$\
(D) $\mathrm { C } _ { 6 } \mathrm { H } 3 \mathrm { Br }$