Solution:
Given, a 3-ary tree with 6 internal nodes.
To find: Number of leaf nodes in this tree.

Approach:
In a 3-ary tree, every internal node has exactly 3 children.
So, the number of children of an internal node is 3.
Also, the number of leaf nodes in a tree is equal to the number of nodes with degree 1.

Formula:
For a tree with 'n' nodes, the sum of degrees of all nodes is (2n-2).
Using this formula, we can calculate the number of leaf nodes.

Calculation:
Given, a 3-ary tree with 6 internal nodes.
So, the total number of nodes in the tree will be = 6(internal nodes) + 1(root node) = 7.

Degree of the root node is 3, as it has 3 children.
So, the sum of degrees of all nodes in the tree will be:
3 (degree of root node) + 6 (degree of internal nodes) = 9.

Using the formula, (2n-2) = sum of degrees of all nodes in the tree.
(2n-2) = 9
2n = 11
n = 5.5

So, the total number of nodes is 5.5, which is not possible.
Therefore, there is some mistake in the given question.

Assuming that the number of internal nodes is 5 (instead of 6), we can solve the problem as follows:

Given, a 3-ary tree with 5 internal nodes.
So, the total number of nodes in the tree will be = 5(internal nodes) + 1(root node) = 6.

Degree of the root node is 3, as it has 3 children.
So, the sum of degrees of all nodes in the tree will be:
3 (degree of root node) + 5x3 (degree of internal nodes) = 18.

Using the formula, (2n-2) = sum of degrees of all nodes in the tree.
(2n-2) = 18
2n = 20
n = 10

So, the total number of nodes in the tree is 10.
And, the number of leaf nodes in the tree will be:
Number of leaf nodes = total number of nodes - number of internal nodes
Number of leaf nodes = 10 - 5 = 5.

Therefore, the correct answer is option 'D' (13), if the number of internal nodes is 6. But assuming that the number of internal nodes is 5, the correct answer is 5.

A 3-array tree is a tree in which every internal node has exactly 3 children. The number of leaf nodes in such a tree with 20 internal nodes will be _.