The correct answer is - a. having differences.
The term heterogeneous is a term used to describe something that is different, that is not the same, or that there's multiple different things. This term can be used for various different things, including human societies, geography, biodiversity, etc.
In the human society, heterogeneous, is used as term when there's societies where multiple different ethnic groups live in a same area, or different racial groups, it may also be used in a manner of representation of the sexes in a particular filed or job, even the architecture and the infrastructure.