Final answer:
The primary rock type underlying the Peninsular Ranges is granite, an igneous rock resulting from the solidification of magma.
Step-by-step explanation:
The primary rock type underlying the Peninsular Ranges is granite. Granite is an igneous rock, which was formed from the cooling and solidification of molten earth material. The Peninsular Ranges are a group of mountain ranges which extend from Southern California into Mexico, forming a part of the Pacific Coast Ranges. Igneous rocks like granite are often found in these types of mountain ranges due to past tectonic processes that involved the cooling of magma beneath the Earth's surface.