Final answer:
Tar sands pose various environmental concerns, including toxic waste in tailings ponds, high greenhouse gas emissions, and risks of oil pipeline spills, making all these factors correct as environmental concerns (D. All of the above).
Step-by-step explanation:
The environmental concerns related to the tar sands involve multiple factors, including the management of waste from tailings ponds which contain a mixture of water, sand, silt, clay, and residual bitumen. Another concern is Greenhouse Gas Emissions; the extraction and processing of tar sands release significant amounts of carbon dioxide and other greenhouse gases. Additionally, Oil pipelines present risks such as leaks and spills that can lead to severe environmental damage. The correct answer to the student's question about what is an environmental concern related to the tar sands is D. All of the above, as each of these aspects poses a threat to the environment.