For a certain reaction R → Product, the plot of concentration [R] versus time has a negative slope as shown. The order of reaction is:

The graph provided shows the concentration of reactant $\mathrm{[R]}$ plotted against time. The line is a straight line with a constant negative slope, indicating that the concentration decreases linearly as time progresses.

In chemical kinetics, the shape of the $\mathrm{[R]}$ versus time plot directly reflects the reaction order:

$Rate\; =\; -\backslash frac\{d[R]\}\{dt\}\; =\; k\; [R]^n$

where $n$ is the reaction order.

Zero‑order reaction ($\mathrm{n\; =\; 0}$):

The rate is independent of the reactant concentration, so $\mathrm{Rate\; =\; k}$. Integrating gives a linear relationship:

$\mathrm{[R]}=\; [R]\_0\; -\; k\; t$

This equation describes a straight line with a negative slope on a concentration‑time plot, exactly as seen in the image.

First‑order reaction ($\mathrm{n\; =\; 1}$):

Leads to an exponential decay: $\mathrm{[R]}=\; [R]\_0\; e^\{-kt\}$, which would appear as a curve that flattens over time, not a straight line.

Second‑order reaction ($\mathrm{n\; =\; 2}$):

Gives a hyperbolic decrease: $\backslash frac\{1\}\{[R]\}\; =\; \backslash frac\{1\}\{[R]\_0\}\; +\; kt$, also not linear.

Fractional order (e.g., 2.5):

Would produce a more complex, non‑linear curve.

Since the observed plot is a straight line with a constant negative slope, the reaction must be zero‑order.

Therefore, the order of the reaction is zero (0).