Just Remember one important formula here,

X = L(N-1) +1

X -> No. of internal nodes

L -> No. of leaf nodes

N -> value of ‘N’ of any N-ary tree.

In the given question it is simply asking to find the Total number of internal nodes in the binary tree because if you delete any internal node of a binary tree then it will split the graph into 2 parts or disconnect the graph.

Binary tree is a 2-ary Tree.

So N=2.

Putting in equation,

X = L(2-1) + 1

X = L + 1 ------> *eqn (i)*

Now we can form another equation from the data given in the question,

It’s given that there are total ‘n’ nodes.

So, No. of internal nodes + No. of Leaf nodes = n

-> X + L = n ---------> *eqn (ii)*

Solving eqn (i) & (ii) for the value of ‘X’,

we get,

X = (n+1)/2

So option C is the correct answer.