Which is true about pop3 and imap for incoming email?
(a)both keep email on an email server by default
(b)imap keeps email on an email server by default
(c)neither keep email on an email server by default
(d)pop3 keeps email on an email server by default.

Both keep email on an email server by default is true about POP3 and IMAP for incoming email

Explanation:

POP3 and IMAP have used setting parameters for incoming email in MS-outlook and lotus domino to receive emails for the end-user.

Normally pop3 will nor keeps mail in emails in the server. once the end-user receives mails for the specific account it deletes emails from the email server.

But IMAP keeps mail in the email server. So the end-user can retrieve the old emails in case he formatted or lost emails.

But email account has some limitation in size. If it is IMAP setting period cleaning should be done in the email server. If it is not done the end-user will not receive any email unless or until deleting the old emails.

Best method is to have pop3 account. But in pop3 still setting can be done in email server not delete for specific period of time.