SKIP ZONE in radio wave transmission is that range where

Question

$\begin{array}{1 1}(a)\;\text{The reception of ground wave is maximum but that of sky wave is minimum}\\(b)\; \text{There is no reception of either ground wave or sky wave}\\(c)\;\text{The reception of sky wave is maximum and ground wave minimum}\\(d)\;\text{Reception of sky wave and ground wave is maximum }\end{array}$