There are 8, 15, 13, and 14 nodes in four different trees. Which one of them can form a full binary tree?

The answer is the tree with 15 nodes. In general, there are 2^n-1 nodes in a full binary tree. By the method of elimination: Full binary trees contain odd number of nodes, so there cannot be full binary trees with 8 or 14 nodes. Moreover, with 13 nodes you can form a complete binary tree but not a full binary tree. Thus, the correct answer is 15.