Answer: a. Carbon monoxide
Step-by-step explanation:
The carbon monoxide gas competes with the oxygen in the blood to occupy the binding affinity of the hemoglobin thus the blood changes it's color to cherry red.
In postmortem lividity, the body becomes discolored due to the effect of flow of blood from the interstitial tissues and suspended under the force of gravity. This can be seen on the dependent parts of the body and the position of body after death. The color of the postmortem lividity depends on the color of the hemoglobin.
In case of carbon monoxide poisoning the color of postmortem lividity appears to be cherry red as color of the hemoglobin is cherry red. Thus cherry red or dark pinkish patches appear in the corpse as lividity.