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.