A model is a representation (or a copy of sorts) of an object or set of objects, but made in a scale that is useful to us. The solar system is not considered a model because it is much too large, and doesn't represent the solar system: it's the real thing. If there were a small mobile of the solar system, then it would be a model.
A planet is not a model because it is not representing something. It IS that something. It is also much too large to handle on a human-size scale.
A padlock is not a model. There is no too-large or too-small object which it represents.
A globe is a good example of a model of earth. It is not a perfectly exact image of earth, but is very useful in considering geography and such. It is also a useful size: most globes can fit on a desk, and some even in the palm of your hand!
Therefore, B. a globe is the correct answer in this situation. Hope this helps!
