a
The arithmetic mean of given values is taken as true value.

\(t _{\text {mean }}=\frac{t_{1}+ t _{2}+ t _{3}+ t _{4}+ t _{5}}{5}\)

\(t_{\text {mean }}=\frac{1.25+1.24+1.27+1.21+1.28}{5}\)

\(t_{\text {mean }}=1.25 s\)

\(\Delta t_{\text {mean }}=\frac{\left|\Delta t_{1}\right|+\left|\Delta t_{2}\right|+\left|\Delta t_{3}\right|+\left|\Delta t_{4}\right|+\left|\Delta t_{5}\right|}{5}\)

\(\Delta t_{\text {mean }}=\frac{0+0.01+0.02+0.04+0.03}{5}=0.02 s\)

Percentage error \(=\frac{\Delta t_{\operatorname{mean}}}{t_{\text {mean }}} \times 100=\frac{0.02}{1.25} \times 100\)

\(=1.6 \%\)