A zero order reaction has a rate law of the form rate = k[A]^0 = k, meaning the rate equals the rate constant and does not depend on the concentration of the reactant. This occurs when some factor other than reactant concentration limits the rate, such as a saturated catalyst surface or a fixed intensity of absorbed light. Because the rate stays constant, the concentration decreases linearly with time according to [A] = [A_0] - kt. Therefore reducing the reactant concentration does not slow the reaction until the reactant is nearly exhausted. The option citing temperature only is incomplete, since temperature affects k for all orders, not the concentration independence. The excess-reactant idea describes pseudo-order conditions, not true zero order. Catalyst control is one possible cause but not the defining reason. This behaviour, seen in surface-catalysed decompositions, is a standard NCERT example. This concept also bridges to Some Basic Concepts in Chemistry and Equilibrium, so mastering it strengthens performance on linked questions from those topics as well. Such questions reward conceptual clarity, since a student who truly grasps zero order reaction can solve many superficially different variants with the same approach. Plausibility check: a constant rate produces a straight-line concentration-time graph, the hallmark of zero order kinetics.