Geologists conclude that the clasts in Layer A are older than the layer itself. Which geologic principle do the geologists use to draw this conclusion?

Principle of inclusion

Explanation:

The principle of inclusion states that "fragments of rocks found in another rock is older than the rock itself".

This principle is very important for relative dating of rocks. It proposes that a rock containing inclusions or fragments of other rocks is younger than the fragments itself. This shows that the rock fragment was already in place before the current rock hosting it formed.

Some rocks can contain fragments of different rock types. The different rock types when encountered are older than the host rock. This is the principle of inclusion.