The given $8:1$ multiplexer output:
$f(x,y,z,w)=\bar x\bar y\bar z I_0+\bar x\bar yzI_1+\bar xy\bar zI_2+\bar xyzI_3+x\bar y\bar zI_4+x\bar yzI_5+xy\bar zI_6+xyzI_7$
$f(x,y,z,w)=\bar x\bar y\bar zw +\bar x\bar yz+\bar xyz\bar w+x\bar y\bar z+x\bar yz++xyz$
$f(x,y,z,w)=\sum(1,2,3,6,8,9,10,11,14,15)$
Solve using k-map we get:
$f(x,y,z,w)=x\bar y+z\bar w+xz+w\bar y$
Option (D) is correct.