The given graph (drawn at constant temperature $T$) is a representation of the kinetics of a reaction. The plot of $y$ versus $x$ is a horizontal straight line parallel to the $x$-axis, i.e. $y$ is constant and independent of $x$. The $y$ and $x$ axes for zero order and first order reactions, respectively, are:

Solution & Explanation

\textbf{Answer:} (C) zero order ($y=$ rate, $x=$ concentration); first order ($y=t_{1/2}$, $x=$ concentration) \textbf{Solution:} A horizontal line means the $y$-quantity does not change as the $x$-quantity changes. For a \textbf{zero order} reaction the rate is independent of concentration: $\text{rate}=k[A{]}^0=k$, so plotting $y=\text{rate}$ against $x=$ concentration gives a horizontal line. For a \textbf{first order} reaction $t_{1/2}=\frac{0.693}{k}$ is independent of the initial concentration, so plotting $y=t_{1/2}$ against $x=$ concentration is also horizontal. Hence zero order: $y=$ rate, $x=$ concentration; first order: $y=t_{1/2}$, $x=$ concentration.