Spamming is often possible because hackers are able to locate and take advantage of which of the following:a. Open relay agent. b. Un-protected systems. c. Protected but open relay agent. d. None of these

a. Open relay agent.

Explanation:

Open relay agent is a protocol of internet configured in such a way that it allows anyone could send emails, this was the initial configuration on the internet. But this protocol is unpopular because of the spammers used for sending a lot of spam.

After a lot of abuse by spammers, operating an open relay came to be frowned upon among the majority of Internet server administrators users.