A department store is a well known retail format. It is one big store, but inside it is divided into separate sections (or "departments") for different kinds of goods, like clothing, electronics, groceries, and furniture, all sold under a single management and a single building. Let's go through the options with this picture in mind.
So a department store simply means a large store made up of many departments under one roof, which makes option (3) correct.
Let's summarize:
The answer is option (3).