Find out that word, the spelling of which is wrong.

The correct answer is C. Harmful.

The word “harmful” is spelled correctly. The other three words are spelled incorrectly.

• “Harmony” is spelled correctly. It means a pleasing arrangement of parts; consonance.
• “Harmonius” is not a word. It is a misspelling of the word “harmonious.” “Harmonious” means having a pleasing or orderly arrangement of parts; consonant.
• “Harmonium” is spelled correctly. It is a musical instrument that produces a sound similar to an organ.