If there are n nodes in a binary tree, the maximum height of the binary tree is n-1 and minimum height is $\left \lfloor log\ 2n \right \rfloor$ .
Read carefully what it is saying :- The height of a binary tree is defined as the number of nodes in the longest path from root to the leaf node. i.e. if node is 3 with (height 2) then height = 3 (a/c to question).
basically it is saying with node = 1 height = 1.
now, for complete binary tree h= ceil(log(n+1)-1) [with height starting from h=0 with node=1.]
hence for this question 8+1= 9.