To be fair this is not a great question. One where the answer in an exam and in reality are not necessarily the same thing! I'd agree with the other comments above and the other answer that it is supposed to be B for the purposes of the test. However, the problem is that many department stores in some countries responded to the growth of shopping malls by sub-letting to other brands/companies (see https://www.business-standard.com/article/companies/retail-majors-cashing-in-on-sub-letting-to-other-brands-107062601003_1.html) and so it's true that they are buildings full of many shops.

B) a shopping mall. A department store is ONE of the many stores inside a shopping mall. Where I live, an outlet store sells the leftover merchandise of a company at a discount. The company could be a well-known brand name.

B-shopping mall! A department store is only one store, a shopping mall is a collection of stores (almost always indoors), and an outlet store is a discount store. A collection of outlet stores in the same place could be called an outlet mall.

A department store is a large store (shop) with different departments (sections) which sell different products such as shoes, clothes, jewelry, cosmetics, books, tools. In North America, it was a predecessor to the shopping mall. It provided one-stop shopping. A shopping mall is collection of independent stores within a large (often enormous) building. A shopping mall often has many shoe stores, clothing stores, and jewelry stores. With the advent of online shopping, many North American malls have closed.