In your experiment, radiation level is the factor that is changed or controlled by the researchers to observe its effect on plant height. Therefore, radiation level is the independent variable.
Consequently, the correct statement is:
Independent variable; graph this on the X axis.
In a typical graph, you would plot the independent variable (radiation level) on the X-axis and the dependent variable (plant height) on the Y-axis, as the plant height is the outcome being measured in response to the changes in radiation level.