The correct answer is A MODEL OF MOUNTAIN FORMATION.
A model refers to a representation of an idea, a process or a system, which is used to explain a specific phenomenon that can not be experienced directly. Models are often used in science to explain many phenomenon. When we are talking about a suitable model to represent an event that happens slowly, the most obvious choice from the options given above is mountain formation. This is because, the process of mountain formation occurs very slowly over a long period of time.
