The correct answer is C) ease of use.
The fact that a web app is written to be run within any of the standard browser and on any platform on which that browser operates is called "ease of use."
The ease of use concept indicates that the product is "amicable" or "friendly" to use by most people because it is not required andy technical knowledge to operate the product. In this case, the app is written to be run within any of the standard browser and on any platform on which that browser operates, so it is "friendly" to operate bu most systems. App's companies and software companies created this "ease of use" concept for many of their products so they can sell most of them because people know they are easy to use and functional.
