Benign (harmless, inoffensive) is an adjective, not an adverb.
A sentence using the adverb, benignly.
"He smiled benignly at the bullies, knowing that he would have his revenge on them in a few hours."
Luckily, the tumor was benign.
Correctly is the adverb in that sentence.
That sentence does not have an adverb.
he was a little benign
He is known to be a benign supervisor.
"we were so happy to find out that John's tumour was benign"Luckily, the tumor was benign.
Subject of the sentence Verb of the sentence Adverb of Manner Adverb of Place Adverb of Frequency Adverb of time Purpose
"Is" is the verb. There is no adverb in the question.
I have several harmless, or benign moles on my skin.
The tumor was benign; it was not harmful for now.
Correctly is the adverb in that sentence.
A tumor was removed from Billy's lung, but thankfully it was benign.