Final answer:
Operatic vocal lines are typically in legato bel canto style against a complex orchestral part, which serves to complement the human voice in opera.
Step-by-step explanation:
Operatic vocal lines are typically in legato bel canto against either a simple or complex orchestral part. However, the option that most closely reflects the traditional partnership between vocals and orchestra in opera is legato bel canto against a complex orchestra part.
When considering the operatic tradition, particularly in the 19th century, bel canto, which means 'beautiful singing' in Italian, was focused on showcasing the human voice with smooth, flowing melodies, or 'legato' lines. The orchestral parts could indeed be complex, serving to complement and underscore the drama and emotion of the vocal line without overpowering it. This style is typified by the operas of composers like Gaetano Donizetti and Vincenzo Bellini.