A $2$x$1$ mux with inputs x$,$y$,$ z and output w can be implemented using assign statement as follow

C

;assign w $=$ z $?$ x:y $.$b O

;assign w $=$ y $?$ x:z $.$c O

;assign w $=$ x $?$ z:y $.$d O

;assign w $=$ x if z else y $.$a