Correct option (a) \(\left(\mathrm{E}^{2} \mathrm{q}^{2} \mathrm{t}^{2}\right) / 2 \mathrm{~m}\)
Explanation:
$$
\begin{aligned}
&\text { Here, } u=0, a=\frac{F}{m}=\frac{q E}{m} \\
&v=u+a t=0+\frac{q E}{m} t \\
&K E=\frac{1}{2} m v^{2}=\frac{m q^{2} E^{2} t^{2}}{2 m^{2}}=\frac{E^{2} q^{2} t^{2}}{2 m}
\end{aligned}
$$