What may be the psuedo code for finding the size of a tree?

(a) find_size(root_node–>left_node) + 1 + find_size(root_node–>right_node)

(b) find_size(root_node–>left_node) + find_size(root_node–>right_node)

(c) find_size(root_node–>right_node) – 1

(d) find_size(root_node–>left_node + 1

The above asked question is from Binary Trees using Linked Lists in chapter Binary Trees of Data Structures & Algorithms I

This question was posed to me in a job interview.