Explanation
Encoding (commonly called "Output Encoding") involves translating special characters into some different but equivalent form that is no longer dangerous in the target interpreter, for example translating the < character into the &lt; string when writing to an HTML page.
Output encoding and input validation are two of the best methods to prevent against this type of attack, which is known as cross-site scripting (XSS). Output encoding is a technique that converts user-supplied input into a safe format that prevents malicious scripts from being executed by browsers or applications. Input validation is a technique that checks user-supplied input against a set of rules or filters that reject any invalid or malicious data. Web-application firewall is a device or software that monitors and blocks web traffic based on predefined rules or signatures, but it may not catch all XSS attacks. Parameterized queries are a technique that separates user input from SQL statements to prevent SQL injection attacks, but they do not prevent XSS attacks. Session tokens are values that are used to maintain state and identify users across web requests, but they do not prevent XSS attacks. Base64 encoding is a technique that converts binary data into ASCII characters for transmission or storage purposes, but it does not prevent XSS attacks.

Explanation
The most likely explanation for what occurred is that the tester IP was blocked by the web server. The web server may have detected the web scanner as a malicious or suspicious activity and blocked the tester's IP address from accessing the web application. This could result in an unauthorized to view this page message in the browser.

A: The libraries may be vulnerable to security bugs or exploits that can compromise the application or the data. According to the web search results, open-source libraries often have vulnerabilities that can be exploited by attackers, such as Heartbleed, Shellshock, DROWN, or npm left-pad1234. These vulnerabilities can allow attackers to extract sensitive data, execute arbitrary commands, decrypt encrypted traffic, or break the functionality of the application. Therefore, using third-party open-source libraries in application code poses a significant security risk.
D: The provenance of code is unknown, meaning that the origin and history of the code are not verified or documented. According to the web search results, open-source libraries and client projects are developed and continuously evolving in an asynchronous way, which makes it difficult to track the changes and updates of the code2. Moreover, open-source libraries may have dependencies on other libraries, which can introduce additional risks or vulnerabilities1. Therefore, using third-party open-source libraries in application code poses a significant quality risk.