The hybridisation of central 'O' atom in $O_3$ molecule is

$\begin{array}{1 1}(a)\;\text{sp-hybridisation}&(b)\; sp^2\text{-hybridisation}\\(c)\;sp^3\text{-hybridisation}&(d)\;dsp^2\text{-hybridisation}\end{array}$

Ozone $O_3$ is in v-shape with $sp^2$-hybridisation
Hence (b) is the correct answer.