A dependent variable is the variable that the experimenter cannot change. The changes it undergoes is 'dependent' on the changes done on the independent variable.
So based on this, the variable that the experimenter cannot control is the length of the shell.
You also can tell which is the dependent variable just by looking at the title of the graph. You are looking at the growth of the Tortoise shell based on the age and whether it came from the wild or from the zoo. You are not changing the length of the shell, but you are looking at how it varies according to certain circumstances.
