To determine the correct answer, let's go through each option and explain why it is right or wrong:

A. has a thin front end (client): This option suggests that using a web application is best when the application has a thin front end or client. A thin front end typically refers to a minimal user interface that focuses on displaying information rather than performing complex operations. In such cases, a web application can be a suitable choice as it can provide a lightweight and easily accessible interface through a web browser. However, this option alone does not cover all the possible scenarios.

B. needs to be available to the public: This option suggests that using a web application is best when the application needs to be available to the public. Web applications can be accessed over the internet, allowing them to be easily accessible to a wide range of users. This option is valid in scenarios where the application needs to be publicly accessible.

C. must be platform-independent: This option suggests that using a web application is best when the application must be platform-independent. Web applications are typically built using web technologies such as HTML, CSS, and JavaScript, which are supported by most modern web browsers across different platforms. This allows users to access the application regardless of their operating system or device, making it platform-independent.

D. Both a and b: This option combines options A and B, suggesting that using a web application is best when the application has a thin front end (client) and needs to be available to the public. Both of these options are valid reasons to choose a web application, but they do not cover the requirement of being platform-independent.

E. All of the above: This option combines options A, B, and C, suggesting that using a web application is best when the application has a thin front end (client), needs to be available to the public, and must be platform-independent. This option covers all the valid reasons stated in the previous options and provides a comprehensive answer.

Based on the explanations above, the correct answer is:

E. All of the above.