To improve the overall quality of web applications, developers should abide by the following rule:

B. Clean and validate all user input.

Explanation:

Option A: Trusting user-supplied data is not a good practice, as it can lead to security vulnerabilities such as injection attacks, cross-site scripting (XSS), and cross-site request forgery (CSRF).

Option B: Clean and validate all user input is a good practice that can help prevent security vulnerabilities. Input validation can help ensure that the data is in the correct format and meets the expected criteria, while input cleaning can help remove any malicious content from the user input.

Option C: Using GET instead of POST is not a rule for improving the overall quality of web applications. GET and POST are different HTTP methods used in web applications, and each has its own advantages and disadvantages. Choosing the right method depends on the requirements of the application.

Option D: Allowing the use of HIDDEN form fields is not a rule for improving the overall quality of web applications. HIDDEN form fields can be used to store data that the user cannot see or modify, but they do not provide any security benefits.

Therefore, the correct answer is:

The Answer is: B. Clean and validate all user input.