d
Addition of one molecule of hydrogen produces cyclohexadiene, in which almost all  of the resonance energy of benzene has been lost. Hence the activation energy of  this step is much greater than that required for each succeeding step, in which the  double bonds behave like their acyclic analogues. Thus we have a multi-step  reaction in which the first step is ratedetermining. This means that the conditions  required for the first step are more vigorous than those required for the successive  steps (all steps involve the addition of one molecule of hydrogen). Because of this, it is not possible to stop the reaction proceeding to complete reduction of benzene to  cyclohexane, and consequently it is not possible to isolate the intermediates.