There are ____ basic types of robots: those that are fixed, and those that are mobile.

In the sentence, the types of robots are being referred to. In the following past, the types mentioned are 'fixed' and 'mobile.' These denote two different types of robots. Therefore, the answer is specific in terms of a number and thus, other options need not be used to describe the types.The sentence is There are two basic types of robots: those that are fixed, and those that are mobile.