From the graph it is evident that the acceleration is decreasing with time.
Also, $a \propto-t$
$\Rightarrow a=-k t \quad$ (decreasing with time)
To find velocity,
$\frac{d v}{d t}=-k t$
$\int d v=\int-k t d t$
$v \propto-t^2$ or graph of velocity should be parabolic with a decreasing slope.