For a complex reaction :

Order of the overall reaction for a complex reaction equals the slowset step's molecular weight. Because the total number of molecules involved in the reaction's slowest stage affects the reaction's overall rate. As a result, the order of the total reaction equals the molecularity of the slowest step.
Because a reaction cannot be completed in the absence of reactants, Therefore, at least one reactant must be present in the first stage of any chemical reaction. As a result, the slowest step's molecularity is never zero or non-integer.

Hence, options A and D are correct.