Final answer:
The correct statement about molecularity is that it has no meaning for complex reactions. Molecularity refers to the number of molecules that collide in an elementary step and can only be unimolecular, bimolecular, or termolecular.
Step-by-step explanation:
The statement that is true about the molecularity of reactions is (b) molecularity has no meaning for complex reactions. The molecularity of a reaction is defined as the number of reactant molecules which collide with one another in an elementary step of a reaction. Molecularity can be unimolecular, involving a single molecule; bimolecular, involving two molecules; or termolecular, involving three molecules. Termolecular reactions are quite rare and are not necessarily fast -- they tend to be very slow due to the improbability of three molecules simultaneously colliding with the correct orientation.
Other statements about molecularity are incorrect: (a) Molecularity cannot be zero or a fraction because it is strictly the count of molecules involved in a single step, and this count must be a whole number. (c) Molecularity is not experimental; rather, it is a theoretical concept derived from the reaction mechanism. (d) While termolecular reactions are indeed rare, when they occur, they are not characterized for being fast.