To construct a $16$ X $1$ multiplexer we need:

Select one:

a$.$ three $2$ X $1$ and one $8$ X $1$ multiplexers

b$.$ two $2$ X $1$ and one $8$ X $1$ multiplexers

c$.$ three $8$ X $1$ and one $2$ X $1$ multiplexers

d$.$ two $8$ X $1$ and one $2$ X $1$ multiplexers