The correct option is Burning of fossil fuels.
Explanation
The concentration of carbon dioxide ($CO_2$) in the Earth's atmosphere has risen dramatically since the onset of the Industrial Revolution. While the carbon cycle involves natural exchanges between the atmosphere, oceans, and biosphere, anthropogenic (human-induced) activities have significantly disrupted this balance, leading to a net accumulation of greenhouse gases.
Analysis of Options:
- Deforestation Deforestation: While deforestation is a major contributor to atmospheric $CO_2$ (accounting for approximately 10-15% of emissions), it acts by reducing the planet's capacity to absorb carbon and releasing stored carbon. However, it is not the largest source compared to fossil fuel combustion.
- Burning of fossil fuels Burning of fossil fuels: This is the primary driver of the rising volume of $CO_2$. The combustion of coal, oil, and natural gas for electricity, transportation, and industrial processes accounts for the vast majority (over 70%) of global anthropogenic $CO_2$ emissions.
- Volcanic eruptions Volcanic eruptions: Volcanoes do release $CO_2$, but the amount is negligible compared to human emissions. Anthropogenic activities release roughly 100 times more $CO_2$ annually than all global volcanic activity combined.
- Melting of polar ice caps Melting of polar ice caps: This is a consequence of global warming rather than a primary cause of rising $CO_2$. While thawing permafrost can release stored carbon (a feedback loop), the melting of ice caps itself is an effect of the warming caused by greenhouse gases.
Key Takeaway:
The burning of fossil fuels is the single largest source of anthropogenic carbon dioxide emissions, far exceeding the contributions from deforestation or natural geological processes.