The most common rule for forming plural nouns is A. adding - es to the word. B. adding - s to the word. C. dropping the last letter and adding - ies. D. changing the y to i and adding - es.

B. The most common rule for forming plural nouns is by adding - s to the singular word which is the simplest among all rules. Examples are given below:

1. Boy (singular), add (s) to make it plural so it become: boys

2. Girl - > girls

3. Book - > books