Final answer:
The least helpful step to alter for changing a reaction's kinetics is a fast step, since the overall rate is determined by the slowest, rate-determining step.
Step-by-step explanation:
The step in a mechanism that would be the least helpful when trying to alter the kinetics of a reaction is (c) a fast step. The rate-determining step is concepts as the slowest step in a reaction mechanism, much like a single slow car on a one-lane road dictates the flow of traffic. Altering a fast step would not have a significant impact on the overall rate of the reaction because the rate is determined by the slowest step or rate-limiting step, not the faster steps. Therefore, focusing on changing a fast step would not effectively change the kinetics of the reaction.