Final answer:
The correct answer is (c) increased greenhouse gas emissions from human activities, which lead to global warming by trapping heat in the atmosphere.
Step-by-step explanation:
Global temperatures have been steadily increasing since the Industrial Revolution largely as a result of increased greenhouse gas emissions from human activities. Answer choice (c) is correct. The rise in carbon dioxide and other greenhouse gases such as methane and nitrous oxide trap heat in the Earth's atmosphere, leading to the phenomenon known as global warming.
These emissions are primarily from burning fossil fuels for energy and transportation, deforestation, and various industrial processes. This increase in global temperatures, also referred to as climate change, is not due to natural climate variability or decreased solar radiation, and while volcanic eruptions can have a temporary cooling effect due to sulfate aerosols reflecting sunlight, they are not the cause of the long-term warming trend.