Answer:
D. The type of music each plant listens to
Step-by-step explanation:
The question indicates each plant is in the same soil (so cannot be C), received the same amount of sunlight (so cannot be B) and the same amount of water (cannot be A).
However, each plant is exposed to a different type of music, one is listening classical, another rock and another country music.
So, the only variable in here is the type of music each plant listens to.