Explanation:

Why Option B is a Security Vulnerability:

This code outputs user-supplied input ($CurrentPage.parameters.userInput) without escaping it (escape="false").
Disabling escaping exposes the page to Cross-Site Scripting (XSS), where malicious scripts can be injected and executed in the user’s browser.
User input should always be escaped or validated to avoid code injection attacks.

Why Not the Other Options:

A.
This uses controller-bound data (ctrl.userinput) and doesn’t display raw URL input.
apex:outputField automatically escapes HTML, so it’s safe.

C.
Similar to A, this uses safe output escaping via outputField.

D.
Though this uses raw user input from the URL, it does escape HTML by default, mitigating XSS.