As a general rule, where can the greatest reductions in an individual's carbon footprint be realized; by replacing incandescent light bulbs with compact fluorescents, bicycling instead of driving, or eating vegetarian?

The answer is; incandescent light bulbs with compact fluorescents

Carbon footprint can be defined as the amount of carbon emissions released into the environment by an individual, organization, or a product.  While cycling will definitely have lower carbon emissions than a car that runs by burning fossil fuels, the greatest reduction in carbon footprint would be felt by replacing incandescent light bulbs with fluorescent bulbs. The latter saves electricity because much of the electricity goes to providing light while in the former, most of the electricity is lost as heat during lighting.