Which county is related to Yonkers and is not part of New York City?

Westchester County is the correct answer because it is the county in which the city of Yonkers is located. Yonkers is situated to the north of New York City and is one of the largest cities within the county. Unlike the other counties mentioned, which either contain boroughs of New York City or are part of the city itself, Westchester County is a distinct political entity that operates independently from New York City. This status highlights the geographical and administrative differences between Yonkers and the cities that are within the New York City boundary.

Richmond, Kings, and Bronx counties are all boroughs of New York City, meaning they are part of the geographical and administrative structure of the city. Therefore, they do not pertain to the question about counties related to Yonkers while being outside of New York City.