Calculate the time in seconds required to reduce the concentration of reactant to half of initial concentration for first order reaction if rate constant is $1.386 \times 10^{-3} \mathrm{~s}^{-1}$

The half-life $t_{1/2}$ for a first-order reaction is determined by $t_{1/2} = \frac{0.693}{k}$
where $k = 1.386 \times 10^{-3}\,\mathrm{s}^{-1}$ is the rate constant.

Substituting yields $t_{1/2} = \frac{0.693}{1.386 \times 10^{-3}} = 500\,\mathrm{s}$.

This corresponds to option D, confirming the answer.