Explanation
The correct answer is B. 22.
Port 22 is the default port for SSH (Secure Shell), a protocol that allows secure and encrypted remote login and command execution over a network. SSH uses public key cryptography to authenticate the client and the server, and to encrypt the data exchanged between them. SSH can be accessed through different ports, depending on the configuration of the server and the client. The default port for SSH is 22, but it can be changed to any other port for security reasons1.
Port 20 is the default port for FTP (File Transfer Protocol) data transfer, a protocol that allows file transfer between a client and a server over a network. FTP uses two ports: one for data transfer (port 20) and one for control commands (port 21). FTP does not provide encryption or authentication, and it is vulnerable to various attacks, such as packet sniffing, spoofing, or brute force. To secure FTP connections, protocols such as FTPS (FTP over SSL) or SFTP (SSH File Transfer Protocol) can be used2.
Port 23 is the default port for Telnet, a protocol that allows remote login and command execution over a network. Telnet does not provide encryption or authentication, and it sends data in plain text, which can be easily intercepted or modified by attackers. Telnet is considered obsolete and insecure, and it should be replaced by SSH or other secure protocols3.
Port 25 is the default port for SMTP (Simple Mail Transfer Protocol), a protocol that allows sending and receiving email messages over a network. SMTP does not provide encryption or authentication, and it can be exploited by spammers or hackers to send malicious or unwanted emails. To secure SMTP connections, protocols such as SMTPS (SMTP over SSL) or STARTTLS (Transport Layer Security) can be used.
Therefore, port 22 is the only port that allows for secure communications by default, as it uses SSH protocol.
For more information about ports and their security, you can refer to the following web search results: How to secure your TCP/IP ports, What is an SSL port? A technical guide for HTTPS, HTTPS Port: What It Is, How to Use It, and More (2023).

Explanation
Once the technician has determined that the power supply is likely to be defective, they should establish a plan of action to resolve the problem. This may include replacing the power supply or further testing to confirm that it is indeed defective. After establishing a plan of action, the technician should implement the solution and verify that it has resolved the issue.

Explanation
The minimum cable category that could be used to support a 10Gbps throughput is Cat 6a. Cat 6a cable is required with 10 Gigabit Ethernet for reliable performance with cable lengths up to 100 metres. However, lower-specification cables can often be successfully used, particularly with the shorter cable lengths required in a typical home123.

Explanation
The correct answer is C. SPF.
SPF stands for Sender Policy Framework, which is a protocol that allows domain owners to specify which IP addresses are authorized to send email on behalf of their domains1. SPF helps prevent email spoofing and phishing by verifying the sender's identity and preventing unauthorized use of their domain name12.
When an email is received, the receiving server checks the SPF record of the sender's domain, which is a DNS TXT record that lists the valid IP addresses for that domain12. The server then compares the sender's IP address with the SPF record and determines if it matches or not. If it matches, the email passes the SPF check and is accepted; if it does not match, the email fails the SPF check and is rejected or marked as spam12.
DKIM stands for DomainKeys Identified Mail, which is a protocol that allows domain owners to digitally sign their emails using a public-private key pair3. DKIM helps ensure the integrity and authenticity of email messages by verifying that they have not been tampered with or forged during transit3 .
MX stands for Mail Exchange, which is a type of DNS record that specifies the mail servers that are responsible for receiving email for a domain. MX records help direct incoming email to the correct destination and prioritize them based on preference values .
DMARC stands for Domain-based Message Authentication, Reporting and Conformance, which is a protocol that builds on SPF and DKIM and allows domain owners to define how receivers should handle unauthenticated or suspicious emails from their domains. DMARC helps protect domains from email fraud and abuse by providing feedback and reports on email delivery and authentication status .
Therefore, SPF is the only service among the options that checks whether the sender's IP address is associated with the sender's domain name when receiving email.

Explanation
The issue described in the question is most likely due to an incorrect configuration of the printer's duplex settings. To enable automatic duplex printing, the technician needs to read the printer's installation manual to understand how to configure the printer's duplex settings correctly. Once the settings are configured, the printer should be able to print on both sides of the paper automatically.
To fix the issue of a newly installed printer capable of automatically printing on both sides of the paper but only printing on both sides manually, the technician should read the installation manual and configure duplex settings. Duplex printing settings can be configured in the printer driver software, which can be accessed through the printer settings in the Control Panel. Installing the most recent firmware upgrade available for the printer, contacting the vendor for a hardware replacement, or reinstalling the printer software and drivers and restarting the printer will not resolve the issue.