Answer:
Layer C is the youngest
Step-by-step explanation:
On the image we can see three layers that have deposited throughout time on top of each other, thus in the manner of superposition. The deeper the layer, the older it is, the closer to the top the layer is, the younger it is, so we have layer D as the oldest, then comes layer B, and the youngest of those three is the layer A. But we also have the layer C that is roughly vertical, and it doesn't seem to belong there. This layer cuts through the layers D and B and stops at the bottom of layer A. This layer is actually an intrusion. It is a magma that managed to rise from the mantle and penetrate through the layers, gradually cooling off afterward and creating intrusive igneous rocks. This layer is the youngest of them all, as it appeared, formed, as the last.