Answer
See answer below.
Work Step by Step
$\alpha$ particles are equivalent to the nucleus of helium: $He^{2+}$, so they are positively charged, have the higher mass among all three and are attracted to the negative charged plate.
$\beta$ particles are equivalent to the electrons: $e^-$, so they are negatively charged and are attracted to the positive charged plate.
$\gamma$ particles are not charged and are not attracted to either charged plate.
