Array-based representation of binary trees - Nodes are stored in an array A - Node v is stored at A[rank(v)] ○ rank(root) = 1 ○ if node is the left child or parent(node), rank(node) = 2 * rank(parent(node)) if node is the right child of the parent(node), rank(node) = 2 * rank(parent(node)) + 1 If this is the case, the both b and c (its labelled d but its the 3rd option) are valid representations. I have drawn out the binary trees below.

Hi! Not sure if you still need this, but the reason why (d) (or (c) in the updated version) is the answer is because it follows the same order as the tree in Q4 (all nodes and subtrees are consistent as the one in Q4). Whereas for (b), the tree is different since the tree has 24 and 34 as children of 13, which was not the case in the tree for question 4. Hope this makes sense :)

is the 13 supposed to not be in the answer? or is it another typo?