The youngest rock unit in the given options is basalt. Basalt is an igneous rock that forms from the rapid cooling of lava. It is commonly found in volcanic areas and is known for its dark color and fine-grained texture.
Shale, sandstone, and limestone are sedimentary rocks that form from the accumulation and compression of sediments over time. They are typically older than igneous rocks like basalt because the process of sedimentation takes longer than the rapid cooling of lava to form rock.
Therefore, in the context of rock units, basalt is the youngest option among shale, sandstone, limestone, and basalt.
