Answer:
Plant D is the control group
Step-by-step explanation:
In an experiment, a control group is the one which is not tested upon by the independent variables in order to compare it with the results obtained from the experiment group (the ones which are acted upon by the independent variable) and hence evaluate the effect of independent variables.
In this case, the independent variables are the different kind of music while the plant A, B and C are the experiment group. Plant D is the control group.
The effect of different form of music on the growth of plant A, B and C is compared with the normal growth of plant D in order to identify which kind of music is good for plant growth.