Final answer:
Layer D could be an intrusion, extrusion, unconformity, or volcano, depending on its relationship with surrounding rock layers and additional characteristics. More context is needed for an accurate identification.
Step-by-step explanation:
The rock feature known as layer D could be identified based on the context provided by the information on geological formations. If layer D is cutting through the other layers, it could be an intrusion, which is formed when magma from the Earth's interior pushes up into existing rock layers and solidifies. However, if layer D is above or below other layers without cutting through them, it might be an extrusion, which forms when lava flows from a volcano and hardens into rock. An unconformity is a gap in the geological record where rock layers have been eroded or not deposited for a period of time before being overlaid by other rocks. Volcanoes are geological structures involved in the eruption process but are not a type of rock layer. Therefore, additional context such as a figure or more specific descriptions of layer D's characteristics would be necessary to provide a definitive answer. To determine if layer D consists of igneous, sedimentary, or metamorphic rocks, one would need to examine characteristics such as texture, presence of fossils, layering and interrelationships with adjacent rock layers, which is often done through stratigraphic analysis.