Which natural process is likely to affect long-term climate changes?

The correct answer is rooted in understanding the natural processes that influence long-term climate changes. Orbital variations refer to the changes in the Earth's orbit and axial tilt over time, which affect the amount of solar energy received by the Earth. These variations occur over tens of thousands to hundreds of thousands of years and are significant in driving ice ages and warm periods in Earth's history.

This contrasts with the other options, which, while they do have major environmental and climate impacts, are largely anthropogenic or human-induced factors. Fossil fuel combustion contributes to greenhouse gas emissions that lead to short-term climate changes, particularly global warming. Urban development alters land use and can create local climate effects but does not play a role in long-term climatic shifts. Deforestation affects local ecosystems and contributes to atmospheric CO2 levels but is also an immediate consequence of human activity rather than a natural process.

Thus, the impact of orbital variations on the Earth's climate is profound and long-term, making it the correct answer in the context of natural processes influencing climate changes over geological timescales.