Hybridisation is found by steric number which is generally the summation of sigma bonds and lone pairs of electrons.

So as we can see $\mathrm{a}$ has four sigma bonds so it has ${\mathrm{sp}}^3$ hybridisation.

$\mathrm{b},\mathrm{c}$ has three sigma bonds so it has ${\mathrm{sp}}^2$ hybridisation.