Choose the formal fronted version of "The backup generator rarely fails during testing."
Rarely the backup generator fails during testing.
Rarely does the backup generator fails during testing.
The backup generator does rarely fail during testing.
Rarely does the backup generator fail during testing.✓Respuesta correcta
Explicación
When rarely moves to the front, the present lexical verb requires does before the subject and fail returns to its bare form. Keeping fails after does would mark agreement twice.