Final answer:
The correct option is d. atmospheric carbon dioxide.
The environmental characteristic most likely to increase due to unchecked human population growth is atmospheric carbon dioxide, which exacerbates climate change and can lead to a reduction in biodiversity and other natural resources.
Step-by-step explanation:
When considering the impact of unchecked human population growth, we can predict that certain environmental characteristics are likely to increase. If the human population continues growing without any constraints, the environmental resource most likely to increase is atmospheric carbon dioxide.
This is because as the population grows, there will be an increased need for energy, which is often produced by burning fossil fuels that release carbon dioxide. Moreover, more people also lead to increased deforestation for agriculture and urban spaces, further contributing to carbon emissions.
Unfortunately, this rise in carbon dioxide levels is associated with negative impacts on the environment, such as climate change, which can lead to a decrease in biodiversity and the depletion of other natural resources like freshwater supplies and arable land. These systems become overstressed, potentially causing them to collapse if the balance between human needs and the earth's capacity to provide is disrupted.