Which word means ominous or suggesting that something bad is going to happen?

The word "portentous" conveys a sense of foreboding or an indication that something significant and often bad is likely to occur. It derives from "portent," which refers to a sign or warning that something momentous or calamitous is about to happen. In literature and conversation, "portentous" often describes events that carry a heavy, sometimes eerie significance, suggesting that the atmosphere is charged with a sense of impending doom.

The other options do not imply this ominous quality. "Benign" refers to something gentle, kind, or not harmful. "Propitious" indicates favorable conditions or success likely to occur, which is just the opposite of anything ominous. "Favorable" also means advantageous or likely to result in success, lacking that negative implication associated with caution or warning. Thus, "portentous" stands out as the word that truly embodies the sense of forewarning, making it the correct choice.