When middle-school-age children understand the rules of language that indicate how words and phrases can be combined to form sentences, this is called understanding?

The answer is syntax. It is because syntax allows forming sentences with the use of arranging phrases and words. It could be classified above as the middle school age children indicates how words and phrases could be combined or arranged with the use of the language that they understood. That is why they are understanding syntax.

C) germany is the anwser