a
\(\begin{array}{l}
From\,F = \frac{R}{{{t^2}}}v\left( t \right) \Rightarrow m\frac{{dv}}{{dt}} = \frac{R}{{{t^2}}}v\left( t \right)\\
Integrating\,both\,sides\,\int {\frac{{dv}}{v} = \int {\frac{{Rdt}}{{m{t^2}}}} } \\
In\,v = - \frac{R}{{mt}}\\
\therefore \,In\,v\, \propto \frac{1}{t}
\end{array}\)