Knowing that all three-sided figures are triangles is an example of:

Concept Formation

Explanation:

The formation of concepts is a process in which specific expressions are taken to make a generalization or a correspondence between things. The formation of concepts is very important in teaching because it is vital to acquire knowledge.

In this case, the fact that all 3-sided figures are triangles is an example of concept formation because a concept is a definition that helps us define if an object is within a certain category.

So for this example, the concept that every three-sided figure is a triangle can help us define when something can be classified or not as a triangle, thus falls into the category of concept formation.