Which sentence has an agreement error triggered by a fraction followed by an of-phrase?
Half of the notebooks is already labelled.✓Respuesta correcta
The damaged roof remains completely hidden by dense fog.
Three bakers carefully measured all the flour.
Several impatient guests are still waiting beside the locked doors.
Explicación
Notebooks is a plural count noun, so half of the notebooks requires are already labelled, not is already labelled. The remaining sentences have ordinary singular or plural subjects whose verbs agree directly with them.