A quick way to work out "malus" is to think of all the everyday English words that start with the prefix "mal-," since nearly all of them carry a negative, damaging sense.
Consider "malfunction," when a machine works badly, "malnutrition," when someone is poorly fed, "malpractice," when a professional acts badly or carelessly, and "malodorous," when something smells bad. In every one of these, "mal-" adds the sense of "bad" or "badly" to the word that follows it. Since "malus" is the Latin word this prefix comes from, its own core meaning has to be the same: bad, or in a slightly stronger everyday word, harmful.
This immediately rules out "comforting" and "mass," which have nothing to do with this "mal-" family, and also rules out "harmless," since that is the exact opposite of what the "mal-" prefix signals in every one of these examples.
Recognizing the "mal-" pattern across common English words points straight to the meaning of the Latin root behind it.
So, the correct answer is harmful.
Since Latin sits behind most modern European languages, comparing "malus" to its direct cousins in languages many students already recognize a little of helps confirm its meaning.
Since every modern language descended from Latin keeps "malus" meaning "bad," the option matching that sense, harmful, is the correct one.
So, the correct answer is harmful.