In which one of these sentences is a prefix used in an italicized word?

C. She was ungrateful for his help.

Among the sentences, this specific statement shows that the italicized word that is ungrateful uses a prefix. The prefix found in the word "ungrateful" is  "un". The prefix "un" means not therefore, ungrateful means not grateful.

According to the “the stand” by stephen king, became the longest book published by king at 1152 pages