The Post Office Protocol 3 (POP3) is a protocol used by clients to download emails from a server. Another possible answer is the Internet Message Access Protocol (IMAP) which is not listed as an option. The Simple Mail Transfer Protocol (SMTP) is not correct as it is only used by clients to send email and by servers to transfer emails from one server to another. SMTP is not used to download email from a server to a client device like a mobile phone.
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common use, and along with IMAP the most common protocols for email retrieval.