CO is harmful for living body due to
It reacts with haemoglobin to form carboxy haemoglobin complex which is about 300 times more stable than oxyhaemoglobin
It reacts with HCl of stomach
It damages nerve
It reacts with N2 to form producer gas
As we have learnt,
CO complex with haemoglobin
Forms carboxyhemoglobin that is highly stable and prevents oxygen carrying capacity of haemoglobin in RBCs, resulting in death.
Therefore, option (1) is correct.