The correct choice, indicating that the intrusion is older, stems from an understanding of geological processes, specifically the nature of intrusions and their relationships with surrounding rocks. An intrusion forms when magma from below the Earth's surface rises and cools, solidifying into igneous rock.
In instances where there is no contact metamorphism observed at the top of the intrusion, it suggests that the surrounding sedimentary layers or rock units were not altered by the heat from the intrusion. This usually occurs when the intrusion predates those surrounding layers, meaning it was already in place before the sedimentary layers were deposited over or around it.
If the intrusion were younger than the surrounding rocks, one would expect some degree of contact metamorphism at the boundary, as the heat from the newly formed rock would impact the adjacent materials. Therefore, the lack of such alteration implies that the surrounding layers were formed subsequent to the intrusion, supporting the conclusion that the intrusion is indeed older than those layers. Understanding these relationships between intrusive igneous rock and surrounding materials is critical in reconstructing geological history and relative ages of different rock formations.