Lets assume,
n0 = number of nodes with 0 child, so no of null branch = $ 2 * n0 $.
n1 = number of nodes with 1 child, so no of null branch = $ n1 $.
n2 = number of nodes with 2 child, so no of null branch = $ 0 $.
Total number of null branch = 2* n0 + n1
We know that, total degree sum of all nodes = 2 * (n-1)
=> 1* n0 + 2 *n1 + 3 * n2 - 1( because except root node, every other node has 1 parent node degree)= 2 ( n0 + n1 + n2 -1)
=> n0 = n2 + 1.
=> n0 = n - n0 - n1 + 1 // n0+n1+n2 = n
2n0 + n1 = n + 1 = total no of null branch.
For n=27, ans should be 28.
Ans - D.
