Final answer:
In Christopher's study on the motion of objects, the dependent variable is the inertia of the object, because the study aims to observe how inertia varies as the mass of an object is changed.
Step-by-step explanation:
The dependent variable in Christopher's study on the motion of objects is the inertia of the object. According to the hypothesis that as the mass of an object increases, its inertia also increases. In this context, the mass is the independent variable which is controlled or varied to observe the effect on the dependent variable, inertia. It's important to note that mass is a measure of the amount of matter in an object and does not depend on gravity, and inertia is the property of an object to resist changes in its state of motion or rest, which is directly proportional to the mass of the object.