Final answer:
An environmental scientist sampling from various US locations is most likely to find plastic pollution, which is pervasive in both terrestrial and aquatic environments and poses significant threats to ecosystems.
Step-by-step explanation:
An environmental scientist taking samples from various locations across the US is most likely to find plastic pollution. Plastics are pervasive pollutants found both on land and in bodies of water, from large visible pieces to microscopic particles. Figures, like those indicating large areas of plastic accumulation in the Pacific Ocean, show the severity of this issue. Additionally, the hazards plastics pose to wildlife and ecosystems are well-documented, often via ingestion or entanglement leading to injuries or death.
While all the listed pollutants — carbon dioxide, heavy metals, plastics, and solar radiation — are environmental concerns, plastics are the type of pollution one can physically collect in a sample, making them more likely to be detected by an environmental scientist in the field. Heavy metals, while a significant problem particularly in industrial and mining areas, might not be as ubiquitous in all types of environments as plastics. Carbon dioxide, though a critical greenhouse gas influencing climate change, is not a substance that can be collected in a sample in the same way solids like plastics or metals can. Solar radiation is a natural phenomenon rather than a pollutant.