Rapid prototyping proves the quality of a design by:

Rapid prototyping is a software-engineering technique for validating a design early: instead of relying on documentation alone, the team builds a limited but functioning version of the target system so its behaviour can actually be exercised, not just described.

In this question, only one offered activity produces such a working model: writing a program that simulates the real system. Running it against realistic inputs lets users and developers directly observe whether the design behaves as intended, which is what proves its quality.

Contrast with the other offered activities:

• Having a system analyst present an overview of the design to users, programmers and consultants is a structured walkthrough/review; useful for communication, but nothing is executed, so it cannot demonstrate behaviour.

• Having a data flow diagram is a static analysis diagram describing data movement; it is documentation, not an executable model.

• Both (a) and (b) extends the simulation with a separate walkthrough activity; since the walkthrough alone does not test behaviour, requiring it does not change what actually proves the design's quality.

Hence rapid prototyping proves design quality specifically by having a program simulate a real system.