Correct answer is C!

To prevent cross-site scripting (XSS) attacks in an Apex controller for a Visualforce site that takes an ID as a URL parameter, the developer should use a method that protects the input from potential XSS attacks.

The best option is:

C. ApexPages.currentPage().getParameters().get('url_param').escapeHtml4()

Justification:

The escapeHtml4() method ensures that special characters in the URL parameter (such as '<', '>', '&') are properly encoded, preventing them from being interpreted as HTML or JavaScript code. This is the safest way to protect your input from XSS attacks.
Other options:

A. String.ValueOf(ApexPages.currentPage().getParameters().get('url_param')): This option simply converts the parameter to a string, but does not protect against XSS.

B. String.escapeSingleQuotes(ApexPages.currentPage().getParameters().get('url_param')): The escapeSingleQuotes method only protects against SQL injections by escaping single quotes, but does not protect against XSS.

D. ApexPages.currentPage().getParameters().get('url_param'): This option offers no protection against XSS and simply retrieves the value of the URL parameter....

Correct answer is B