$Ca(OH)_2+Cl_2\rightarrow CaOCl_2+H_2O$.This reaction can be used in

$\begin{array}{1 1}(a)\;Nelson\;process\\(b)\;Bechman's\;plant\\(c)\;Hyber\;process\\(d)\;Whytlaw\;Gray\;method\end{array}$

Preparation Bleaching powder
$Ca(OH)_2+Cl_2\rightarrow CaOCl_2+H_2O$
Hence (b) is the correct option.