Explanation

(a) Acceleration due to gravity on the surface of the earth is
$g_{e} = \frac{GM_{e}}{R_{e}^{2}}$
Where $M_{e}$ and $R_{e}$ are the mass and the radius of the earth respectively
Acceleration due to gravity on the surface of the planet is $g_{p} = \frac{GM_{p}}{R_{P}^{2}}$
Where $M_{p}$ and $R_{p}$ be the mass and the radius of the planet respectively
If both mass and radius of the planet are half as that of the earth, then
$g_{p} = \frac{G(M_{e}/2)}{\left( R_{e}/2 \right)^{2}} = 2\frac{GM_{e}}{R_{e}^{2}} = 2g_{e}$