The correct answer is D, A=Trophozoite; B=immature cyst; C= infective cyst.  Balantidium coli is a widely distributed ciliate protozoan found in humans, monkeys, and pigs.  During its life cycle, the invasive trophozoite of this protozoan represented by diagram 1, emerges from the cyst.  Encystation of trophozoites leads to production of infective cysts that are shed in the stools, represented by diagram 3.  The stage in diagram 2 represents an immature cyst in the initial phase of encystation.

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