I disagree. POP (110) and IMAP (143) are used in receiving and retrieving emails, respectively. The question clearly asks which port is to be left open so the server can relay/send emails. This is clearly the function of SMTP (25). https://www.siteground.com/tutorials/email/protocols-pop3-smtp-imap/

This one is simple. Shouldn't even be debated

Port 25 for relaying emails

Port 25 is the default port for relaying email on the internet. However, you can specify any port number (Provided it is within the acceptable range of custom port numbers) for submitting emails to your SMTP server. There are special cases when you must use port 25 on your SMTP server.

key words here are "relay email". Port 25 is the answer.

SMTP can be used to both send and receive mail. Post Office Protocol version 3 (POP3) and Internet Message Access Protocol version 4 (IMAP4) can be used only to receive mail.

Step #1: After composing a message and hitting send, your email client - whether it's Outlook Express or Gmail - connects to your domain's SMTP server. This server can be named many things; a standard example would be smtp.example.com. Step #2: Your email client communicates with the SMTP server, giving it your email address, the recipient's email address, the message body and any attachments. So answer is port 25 which is B

This one kinda throws you off as it says default ports. And it doesn't tell you to select 1 or multiple answers