Answer: B, the second line: <apex:outputText escape="false" value="{!$CurrentPage.parameters.userInput}" />

I’m assuming the options A to D follow the four lines in the order shown in your screenshot.

This line is vulnerable to cross-site scripting (XSS). escape="false" turns off HTML encoding, and the value comes straight from a URL parameter that the user controls. An attacker can craft a link such as ?userInput=<script>...</script>, and the script would run in the victim’s browser.

The other lines are safe:

A: outputText HTML-encodes its value by default (escape="true"), so any script in the parameter is displayed as text and not executed.
C and D: outputField renders the value according to its field type and also encodes it. The rendered attribute in D only controls whether the component appears.

How to fix it:

Leave escape at its default of true unless you really need raw HTML.
If you do need to output markup, sanitize the input in the controller first, or use JSENCODE, HTMLENCODE or URLENCODE as appropriate for where the value is placed....

Correct answer is
<apex:outputText escape="false" value="$CurrentPage.parameters.userInput" />