b
\(\begin{array}{l}
\,\,\,\,\,\,\,\,\,\,{P_0} = Fv\\
\therefore \,F = ma = m\frac{{dv}}{{dt}}\therefore \,{P_0} = mv\frac{{dv}}{{dt}}\,\\
or\,{P_0}dt = mvdv\\
Integrating\,both\,sides,\,we\,get\\
\int\limits_0^t {{p_0}dt = m} \int\limits_0^v {vdv} \\
{p_0}t = \frac{{m{v^2}}}{2}\\
v = {\left( {\frac{{2{p_0}t}}{m}} \right)^{1/2}}\,\,\,\,or\,\,\,\,v \propto \sqrt t
\end{array}\)