Step 1: Recall what a supermarket is:
A supermarket is a large self-service store. It sells food, grocery and other household goods in separate aisles.
The question says its size is neither too small nor too big. So it lies between a convenience store and a hypermarket.
Step 2: Place the formats on a size scale:
A convenience store is the smallest. It usually has only a few thousand square feet or less.
A supermarket is medium in size.
A hypermarket is the largest and goes up to several tens of thousands of square feet.
Step 3: Check the options:
5,000 sq. ft. is the size of a small convenience or neighbourhood store, so it is too small as an upper limit.
50,000 sq. ft. is the scale of a hypermarket, so it is too big.
30,000 sq. ft. is also on the large side for a supermarket.
20,000 sq. ft. is the medium figure that the textbook gives as the usual upper limit for supermarkets.
Final Answer:
Supermarkets are generally up to 20,000 sq. ft., which is option (B).