A first order reaction is $50 \%$ completed in 16 minute. Find the percentage of reactant reacting in 32 minute.

The half-life $t_{1/2}$ of a first-order reaction is constant and equal to the time required for the reaction to reach 50% completion. Given that this occurs in 16 minutes, we have $t_{1/2} = 16$ minutes.

A time period of 32 minutes corresponds to $\frac{32}{16} = 2$ half-lives. The fraction of reactant remaining after $n$ half-lives is $\left(\frac{1}{2}\right)^n$, yielding $\left(\frac{1}{2}\right)^2 = \frac{1}{4}$ remaining.

The percentage of reactant reacted is therefore $100\% - 25\% = 75\%$.

Final answer: D