There is a minor difference between the 2 questions..In the previous year gate question , it is asked that :
Minimum size of an array required to store any binary tree with 'n' nodes
So we have to consider all sorts of binary trees ..Hence we are considering worst case which is skew binary tree which takes space for n nodes = 2n - 1
But in the question mentioned here , it says :
Minimum size an array may require to store a binary tree
So it is actually a weaker statement than the previous one..It considers the best case scenario actually..It is not considering all binary trees as it is not saying "any binary tree"
So here we consider the best case which corresponds to complete binary tree..Hence n no of spaces required in an array for storing a binary tree..So the possible minimum size that an array may need to store a binary tree is 'n' only..
Hence A) is the correct answer..