The correct answer is 'https://www.example.com/product.php?id=1234&category=tools'. This URL contains parameters ('id' and 'category'), which can be tested for vulnerabilities such as SQL injection, XSS, and more. A penetration tester could manipulate these parameters to see how the application responds, thereby potentially discovering security flaws.

The incorrect answers listed don't provide the same level of actionable information. The URL containing the 'mailto' protocol is typically used for email and does not usually have parameters that could be tested for web application vulnerabilities. The URL with 'https://www.example.com/privacy' is likely a static page and while it could contain potential endpoints for further investigation, it does not explicitly showcase parameters like the correct answer. Lastly, the URL 'ftp://ftp.example.com/resources' uses the FTP protocol, which is less likely to be the focus of this type of testing in comparison to HTTP(S), which directly interacts with web applications.