Solve this:

Dear Student,

The rate of reaction can be given as:
$$\begin{gathered} Rate of reaction = -\frac{1}{2}\frac{d\left[HI\right]}{dt}=\frac{d\left[H_2\right]}{dt}=\frac{d\left[I_2\right]}{dt} \\ Now, d\left[HI\right] = Final concentration of HI - Initial concentration of HI = 0.4 - 0.5 = -0.1 \\ dt = time period = 10 min \\ Thus, \\ Rate of reaction = -\frac{1}{2}\frac{d\left[HI\right]}{dt}=-\frac{1}{2}\frac{(-0.1)}{10}=5 \times {10}^{-3} M mi{n}^{-1} \\ Thus, the correct answer is option \left(a\right). \end{gathered}$$

Hope it helps.

Regards