c
Enzyme -catalysed reactions follow first order kinectics.

therefore, Fall of concentration from \(1.28 \;\mathrm{mg}\, \mathrm{L}^{-1}\) to \(0.04\; \mathrm{mg} \,\mathrm{L}^{-1}\) involves five half-lives.

\(1.28 \stackrel{t_{1 / 2}}{\rightarrow} 0.64 \stackrel{t_{1 / 2}}{\rightarrow} 0.320 .16 \stackrel{t_{1 / 2}}{\rightarrow} 0.08 \stackrel{t_{1 / 2}}{\rightarrow} 0.04\)

therefore, Time required \(=5 \times t_{1 / 2}\)

\(=5 \times 138\; \mathrm{s}\)

\(=690\; \mathrm{s}\)