Choose the word which is closest in meaning to the word in italics (underlined) in the sentence. It was…

The correct choice is: “Insulting”

Scurrilous means grossly abusive, insulting and defamatory, meant to damage someone’s reputation. A scurrilous attack is an insulting one.

A scurrilous attack is often also unfair, but “unjustified” only says it lacks justification and misses the abusive, insulting nature that the word describes. “Serious” and “justified” do not capture the meaning at all.