Question

Which sentence has a capitalization error

Answer 1

Number 4: She has six Aunts, five Uncles, and fifteen cousins, has a capitalization error.

Aunts and Uncles should be lower cased because they are common nouns. They describe the people who are the aunts and uncles. However, if you had a name that followed aunt (like in number 5), then Aunt and the name of the person would be capitalized because it is a title of a person and it becomes a proper noun.

Hopefully this helped. 

Answer 2

In (4) aunts is a common noun and doesn't need to be capitalized.
In (5) it's used as a title together with a name and so ,it should be capitalized