Answer: model
Explanation:
When a scientist is against a very complex idea, one of the steps that the scientist takes is simplify (for example, hey may suppose that during the process the temperature is constant, or things like that) as much as him/her can. The objective here is creating a model from where he can start to predict the behavior of the object or the processes, and then with scientific analysis and experimentation the scientist can, first probe or refute his/her model, and if the model seems to work, with the further investigation this model can be perfectioned.
The complete sentence is:
Earth scientists often use a model to represent complex objects or processes.
