Carbon monoxide (CO) is a highly toxic gas that binds to haemoglobin in the blood with an affinity much greater than oxygen. This forms carboxyhaemoglobin, which effectively reduces the amount of oxygen that the blood can transport to the body's organs and tissues, leading to tissue hypoxia (oxygen deprivation).

A. Major cause of respiratory tract infections: Carbon monoxide does not cause infections; it is a chemical asphyxiant.

C. Preventing clotting of blood: Carbon monoxide does not interfere with the blood clotting process (coagulation); it primarily affects oxygen transport.

D. Causing gene mutation: While some chemicals can cause gene mutations (are carcinogenic or mutagenic), this is not the primary or immediate effect of carbon monoxide poisoning. Its main harm is through oxygen deprivation.

The correct answer is reducing oxygen carrying capacity of the blood - option B.