Answer:
Exposure to music or Music
Step-by-step explanation:
The independent variable is the variable in an experiment that is changed to determine if the change will have an effect on the subject. With the title alone, "The effects of music on the growth of tomatoes", we can see tht Jordan wants to see the effect of music. To test this, music is changed to determine if there is any difference.
On one plant classical music is played and on the other, no music is played. So what was changed is the music or exposure to music.