The correct answer is option 2: ability to form a complex with haemoglobin.

Carbon monoxide, also known as CO, is a highly toxic gas that can be deadly in high concentrations. It is often referred to as a silent killer because it is odorless and colorless, making it difficult to detect without proper equipment.

Option 1: Insolubility in water is not the main reason for the poisonous nature of carbon monoxide. While carbon monoxide is indeed only slightly soluble in water, its toxicity is primarily due to its ability to bind with haemoglobin in the blood.

Option 2: The main reason for carbon monoxide`s toxicity is its ability to form a strong bond with haemoglobin, the protein responsible for carrying oxygen in the blood. When carbon monoxide binds to haemoglobin, it forms carboxyhemoglobin, which reduces the blood`s ability to transport oxygen to tissues and organs. This lack of oxygen can lead to serious health effects, including tissue damage and organ failure.

Option 3: While carbon monoxide can interact with certain metal oxides, this is not the primary reason for its poisonous nature. The formation of carboxyhemoglobin with haemoglobin is the main mechanism through which carbon monoxide ex