The rate of reaction was found to be equal to its rate constant at any concentration of the reactant. The order of the reaction is:

Zero-order

The correct answer is option 1. Zero-order.

In chemical kinetics, the order of a reaction indicates how the rate of the reaction depends on the concentration of reactants. Specifically, it describes the relationship between the rate of reaction and the concentration of reactants.

For a zero-order reaction, the rate of reaction is independent of the concentration of the reactant(s). This means that even if the concentration of the reactant(s) changes, the rate of the reaction remains constant. Mathematically, the rate equation for a zero-order reaction can be expressed as:

\(\text{Rate} = k \)

where:

\( \text{Rate} \) is the rate of the reaction,

\( k \) is the rate constant.

In a zero-order reaction, the rate constant \( k \) is equal to the rate of the reaction. This implies that the rate of the reaction is constant and does not change with changes in concentration.

Therefore, if the rate of reaction is found to be equal to its rate constant at any concentration of the reactant, it indicates a zero-order reaction.

In summary, the observation that the rate of reaction is equal to its rate constant at any concentration of the reactant(s) suggests that the reaction is zero-order. This means that the rate of the reaction is independent of the concentration of the reactant(s), leading to a constant rate of reaction regardless of changes in concentration.