Answer:
The correct answer is - plant B.
Step-by-step explanation:
Control group or factor is the factor or variable that did not get any treatment in an experiment that rest test subjects are getting so the result can be compared to the control group.
Plant b is a control group here as it did not get the music as a plant A so the effect of music on plant A can be compared to plant B and see if there is any change occurs.
Thus, the correct answer is - plant B.