Final answer:
The characteristic musical form of Schoenberg's Pierrot lunaire is Theme and variations, despite the work's varied and flexible atonal style.
Step-by-step explanation:
The musical form that is characteristic of Arnold Schoenberg's Pierrot lunaire is C) Theme and variations. Pierrot lunaire is a melodrama consisting of 21 pieces of music and is one of the landmarks of the composer's atonal period. It does not consistently follow classical forms such as sonata-allegro, minuet and trio, or rondo. Instead, Schoenberg employs a free atonal style with each piece having its unique form, though some suggest that overarching theme and variations and rondo-like structures can be gleaned when looking at the work as a whole.