。
Are you seriously saying that?
Literal
Seriously so say-[ongoing-contracted-てん]-[explanation-の]-[question-か]?
言ってん is a colloquial contraction of 言っている (の), where 言っている → 言ってる → 言ってん before の. The のか ending forms a challenging, incredulous question. Very casual and somewhat confrontational — the kind of thing said when you can't believe what you just heard.