Answer: Length of the shell (mm)
Step-by-step explanation:
An independent variable is the one which can be controlled or changed in an experiment to test it's effect on the dependent variable. The dependent variable can be define as the variable which can be tested or measured in an experiment.
Here, in the given graph the length of shell(mm) is the dependent variable which depends upon the age in years. As with the age of the tortoise the length of the shell will also increase.