question is ambiguous Z is unspecified here if we consider it 2 then e.g if we consider a heap of element 7 (complete binary tree with height h) then nodes at height h will be 4 .
At height h the no of nodes may vary from 1 to 2^h
now if we consider Root as node of height H then it will always be 1 and option D gives this ans
as ceil(n/2^h+1) will be upgraded to 1