Using $8*1$ mux we can implement all possible functions of $3$ variable. if we want to obtain function of $4$ variable then along with $3$ selection lines ,we have to use some of $8$ input lines for forth variable, through which we can implement some of $4$ variable function not all.
Hence, option C is correct.
https://www.youtube.com/watch?v=bEnWYiPNtZk