Step 1: Nature of molecularity.
Molecularity is a theoretical quantity defined as the number of reacting species that must collide simultaneously in a single elementary step. It is always a positive whole number and cannot be zero or fractional.
Step 2: Molecularity vs. order.
Reaction order is determined experimentally from the rate law. Molecularity, in contrast, is a theoretical concept derived from the proposed mechanism, not from experiment. Option (A) incorrectly calls molecularity experimental.
Step 3: Complex reactions.
A complex (multi-step) reaction has no single overall simultaneous collision. Since molecularity applies only to individual elementary steps, it has no meaning for the overall complex reaction. Option (B) correctly states this.
Step 4: Eliminate other options.
Option (C) is wrong because molecularity must be a whole number. Option (D) is wrong because reactions with four or more simultaneous collisions are essentially impossible.
\[ \boxed{(B)} \]