Phrasal verb – harmony vs. quarrel: supply the particle that completes "fall ____" to mean "quarrel". Sentence: "Brothers must live in harmony. They must never fall ____." Choose the correct particle to complete the phrasal verb.

Given data

• The phrasal verb "fall ____" should convey "to quarrel, to stop being friendly".

Concept/Approach
Fall out (with someone) = have a quarrel or disagreement; relationship breaks down.

Option analysis
out — correct particle: "fall out." ✅apart — means disintegrate physically/emotionally, not specifically "quarrel".off — means decrease or depart; not the idiom for quarrelling.down — means collapse or reduce; not used here.

Usage example
"They fell out over money."

Final Answer
out