Final answer:
The reasonable unit for the rate constant of a second order reaction is mol/L.sec.
Step-by-step explanation:
The units for the rate constant of a second order reaction can be determined by following dimensional analysis. Since the overall order of the reaction is 2, the units of the rate constant would be L(a) mol(b)-2 c-1, where a, b, and c are integers that satisfy the equation a + b + c = 1. Among the given options, the unit that fits this description is A. mol/L.sec. This corresponds to L(1) mol(0)-2 s(0)-1. Therefore, A.mol/L.sec is the reasonable unit for the rate constant of a second order reaction.