This is my brother’s book.

In the given sentence, the word brother’s is the possessive noun.

As the word brother is singular, the apostrophe is added before s.

So, the singular possessive noun form for the word brother is brother’s.

The kid’s toy is colorful.

In the given sentence, kid’s is the possessive noun.

As the word kid is singular, the apostrophe is added before s.

So, the singular possessive noun form for the word kid is kid’s.

He cleaned the dog’s house.

In the given sentence, dog’s is the possessive noun.

As the word dog is singular, the apostrophe is added before s.

So, the singular possessive noun form for the word dog is dog’s.