Answer
$0.109 \mu s$
Work Step by Step
Given: $m= 4.00\times1.661\times10^{-27}kg$
$q= 2\times1.602\times10^{-19}C$ and $B=1.20 T$
Recall: Period of revolution T=$\frac{2\pi m}{qB}$
Substitute: $T= \frac{2\pi\times4.00\times1.661\times10^{-27}kg}{2\times1.602\times10^{-19}C\times1.20 T}$
Result: $T= 1.09\times10^{-7} s= 0.109 \mu s$
