Which sentence contains the meaning error in the optionality-versus-prohibition contrast?
The solvent is highly flammable, so workers mustn't smoke near the open drum.
Breakfast is included but optional, so guests don't have to order it.
Friday's extra rehearsal is optional, so choir members mustn't attend it.✓Bonne réponse
The nesting area is closed, so walkers mustn't leave the marked path.
Explication
The choir sentence is wrong because mustn't attend turns an optional rehearsal into a ban. Don't have to attend is required when singers may choose freely whether to come.