Which one of the following is corrrect example of the plural possessive case?

A singular noun forms the possessive by adding and apostrophe s ('s) to the end of the Word: brother's
Example: I borrowed my brother's car.

There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the Word: boss'
Add an apostrophe s ('s) after the existing s at the end of the Word: boss's
Examples:
I borrowed the boss' car.
I borrowed the boss's car.