complete binary tree is filled left to right
and we know that maximum number of nodes possible in complete binary tree of height h = $2^{h+1}-1$
here,
option A) $2^0+2^1+2^2+2^3+......+2^{h-1}+2^h = 1 + 2(\frac{2^h-1}{2-1}) = 1 + 2^{h+1} - 2$
$= 2^{h+1} - 1$