Among the options listed, eating less red meat so fewer cattle need to be raised would help reduce global warming.
Livestock production, particularly cattle, is a significant source of greenhouse gas emissions, including methane, which has a much higher warming potential than carbon dioxide. Reducing red meat consumption can help lower the demand for cattle farming, which in turn can decrease these emissions and contribute to mitigating climate change.
The other options—riding a bicycle rather than walking, clearing forests for farmland, and using more fertilizers—either have minimal impact on reducing global warming or could potentially worsen the situation (e.g., clearing forests contributes to increased carbon emissions).