The node is the only mode that has no parent
a. root
b. binary
c. child
d. n-ary

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.

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