b
\((b)\) On the basis that the rate determining step is the formation of the  carbonium ion, then, with benzene as the reference compound, the rate is  faster when the substituent activates the ring ( \(+I \)and \(I\) or \(+R ; o/ p\)) and  the rate is slower when the substituent deactivates the ring (\(-I\) and \(I\) or \(- R ; m\)). Halogen is an exception; it deactivates the ring \((-I, + R ; - I > +R)\), but the orientation is \(o/ p.\) Hence, the order of increasing rate of nitration is : 
                                  \(PhNO_2 < PhCl < PhH = C_6D_6 < PhMe\)
                                  \((-I, -R)\)       \((-I, -R ; -I > +R)\)           \((+I)\)
Since proton removal from the carbonium ion is fast, no kinetic isotope -effect  is observed when \(H\) is replaced by \(D\). Hence, the rates of nitration of \(PhH\)  and \(C_6D_6\) are the same. Had a kinetic isotope effect been  observed, this would mean that the second step had now become rate  determining.