Explanation

(b) Because of large mass and large velocity, $\alpha$-particles have large ionising power. Each $\alpha$-particle produces thousands of ions before being absorbed. The $\beta$-particles ionise the gas through which they pass, but their ionising power is only $\frac{1}{100}\ $th that of $\alpha$-particles.$\ \gamma$-rays have got small ionising power.
Because of large mass, the penetrating power of $\alpha$-particles is very small, it being 1/100 times that to$\beta$-rays and 1/10000 times that of $\gamma$-rays. $\alpha$-particles can be easily stopped by an Aluminium sheet, only 0.02 mm thick. $\beta$-particles have very small mass, so their penetrating power is large. $\gamma$-rays have very large penetrating power.