For a complex reaction,

I. Order of overall reaction is same as Molecularity of the slow step.

II. Order of overall reaction is less than the Molecularity of the slowest step.

III. Order of overall reaction is greater than Molecularity of the slow step.

IV. Molecularity of the slowest step is never zero or non-integer.

(a) For a complex reaction, order of overall reaction = molecularity of slowest step as rate of overall reaction depends upon total number of molecules involved in slowest step of the reaction. Hence, molecularity of the slowest step is equal to order of overall reaction.
(b) Completion of any reaction is not possible in the absence of reactants. Hence, slowest step of any chemical reactions must contain at least one reactant. Thus, molecularity of the slowest step is never zero or non- integer.