Some of the members of the Board indulged in highly scurrilous talk at the meeting.

Answer: Option C

Irrelevant : not connected with or relevant to something.
Superfluous : unnecessary, especially through being more than enough.
Abusive : extremely offensive and insulting.
Objectionable : arousing distaste or opposition; unpleasant or offensive.
Scurrilous : making or spreading scandalous claims about someone with the intention of damaging their reputation.

Synonym of Scurrilous is Abusive