For a certain chemical reaction, the plot of half-life ( t_ 1/2 ) versus the initial concentration of the reactant ( [R₀] ) yields a horizontal straight line parallel to the concentration axis. The order of this reaction is:

Step-by-step solution

For a zero-order reaction, the half-life is directly proportional to the initial concentration: t_ 1/2 = [R₀] 2k . For a first-order reaction, the half-life is independent of the initial concentration: t_ 1/2 = 0.693 k . Since the plot of t_ 1/2 versus [R₀] is a horizontal line parallel to the concentration axis, it indicates that t_ 1/2 is constant and does not change with [R₀] . This independence from initial concentration is a characteristic feature of a first-order reaction. Answer: 1