The correct answer is Global warming.

Global warming refers to the long-term heating of Earth's climate system observed since the pre-industrial period (between 1850 and 1900) due to human activity,primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth's atmosphere. The term is frequently used interchangeably with the term climate change, though the latter refers to both human- and naturally produced warming and the effects it has on our planet. It is important to distinguish global warming from the natural greenhouse effect, which is a necessary process for life on Earth. The natural greenhouse effect keeps Earth's average temperature at approximately 57 degrees Fahrenheit (14 degrees Celsius), whereas human activities have significantly amplified the effect, leading to the observed warming trend.

The other options are incorrect because:

• Ozone depletion: This refers to the thinning of the ozone layer in the Earth's stratosphere, primarily caused by the release of chlorofluorocarbons (CFCs). While related to environmental issues, it is not directly the cause of the increased average temperature of the Earth's atmosphere.
• Acid rain: This is caused by the release of sulfur dioxide and nitrogen oxides into the atmosphere, which react with water to form acidic precipitation. While it's a significant environmental problem, it's not directly related to the gradual increase in average global temperature.
• Deforestation: While deforestation contributes to climate change by reducing the planet's capacity to absorb carbon dioxide, it is not the primary term describing the increase in average global temperature caused by greenhouse gases.