In a full binary tree if number of internal nodes is I, then number of nodes N are?

(a) N = 2*I

(b) N = I + 1

(c) N = I – 1

(d) N = 2*I + 1

This interesting question is from Binary Tree Properties in chapter Binary Trees of Data Structures & Algorithms I

The question was asked by my school teacher while I was bunking the class.