Answer:
A. Root
Explanation:
A root node is a node which has no parent. All other nodes have a parent and zero or more children. Root node is the only node for which parent is null.
As an example consider the following tree structure:
Node R has 2 children C1,C2 . C1 in turn has 2 children C11, C12 . C2 again has two children C21 and C22. As we can see, each node C1,C2, C11,C12,C21,C22 has well defined parents with the only exception of the root R.
