Final answer:
Scenario b, depicting a glacier carrying sediments that cause eutrophication in a lake, is an example of natural pollution, resulting from natural processes rather than human activity.
Step-by-step explanation:
Within the given scenarios, scenario b, where a melting glacier picks up sediments and deposits them in a lake, causing eutrophication, would be considered as natural pollution. This is because it results from natural processes rather than human activity. In this case, the sediments could contain nutrients like phosphates and nitrates that may support the growth of algae and microorganisms, potentially leading to eutrophication. Eutrophication is the enrichment of an ecosystem with chemical nutrients, typically compounds containing nitrogen or phosphorus, which may lead to excessive plant growth and decay, and the lack of oxygen in a water body. This can have negative effects on aquatic life and water quality.
Human activities such as dumping garbage, even if biodegradable, or improper disposal of chemicals like coolant and motor oil, are not natural but rather anthropogenic sources of pollution. Natural events like volcanic eruptions, storms, and the natural emissions from flora and fauna also contribute to the total pollution level, but they are not directly caused by human activities.