Question

What is the possessive noun for the sentence,
The girls parents were also close friends.

Answer 1

girls is the possessive noun bc its taking a singular noun and making it with more (girl=1, girls= 2) 

Answer 2

The possessive noun in this sentence should be used with the word "girls", following the next format:

The girls' parents were also close friends.

This example indicates that the parents belong to the girls: they are the parents of the girls.

If the sentence talks about only one girl, the correct format to indicate the possessive would be:

The girl's parents were also close friends.