This is a sink for CO

The correct option is "Micro organisms present in the soil".

Understanding a Carbon Monoxide (CO) Sink:
In environmental chemistry, a "sink" refers to a natural or artificial reservoir that absorbs and stores a chemical compound or pollutant from the atmosphere, thereby reducing its concentration. Carbon monoxide (CO) is a toxic, colorless, and odorless gas primarily produced by the incomplete combustion of fossil fuels, biomass, and other organic matter.

Why Microorganisms in the Soil Act as a Sink:
The soil contains a vast and diverse community of microorganisms, including specific bacteria (such as carboxydotrophic bacteria) and fungi. These microbes possess specialized enzymes that enable them to metabolize carbon monoxide. They utilize CO as a source of carbon and energy, converting it into less harmful substances like carbon dioxide (CO₂) and methane (CH₄), or incorporating the carbon into their cellular biomass. Through this biological activity, soil microbes actively remove large quantities of CO from the atmosphere, making soil one of the most significant natural sinks for carbon monoxide.

Analyzing the Other Options:
1. Haemoglobin: While carbon monoxide binds strongly to haemoglobin in the bloodstream to form carboxyhaemoglobin, this is a toxicological reaction occurring inside living organisms (causing oxygen deprivation) rather than a global environmental sink that cleanses the atmosphere.
2. Oceans: Oceans are a major sink for carbon dioxide (CO₂), but they actually act as a minor net source of carbon monoxide (CO) rather than a primary sink, due to the photodegradation of organic matter in the surface waters.
3. Plants: Plants primarily absorb carbon dioxide (CO₂) for photosynthesis. While they can take up trace amounts of CO, they do not serve as a primary global sink for it.