The carbon footprint can be decreased by a great amount if the incandescent bulbs are replaced with the compact fluorescent bulbs. The compact fluorescent bulbs uses approximately seventy five percent less energy than the incandescent bulbs as they do not use heating mechanism to light. This leads to less emission of the green house gases and thereby reducing the carbon footprint.
Hence, the correct answer is 'option a - replacing incandescent bulbs with compact fluorescent bulbs'.