Explanation

(b) Momentum $(p)$ is defined as product of mass $(m)$ and velocity $(v)$
$p = mv$
$p_{1} = {mv}_{1},\ \ \ p_{2} = m\left( 2v_{1} \right)$
$\therefore\frac{p_{1}}{p_{2}} = \frac{m\ v_{1}}{2m\ v_{1}} = \frac{1}{2}$
$\Longrightarrow \ \ \ p_{2} = 2\ p_{1}$
Which implies that momentum is doubled?