The gas mixture used by deep sea divers for respiration is:
(A) ${{\text{O}}_{\text{2}}}{\text{ + }}{{\text{N}}_{\text{2}}}$
(B) ${{\text{O}}_{\text{2}}}{\text{ + He}}$
(C) ${\text{Xe + }}{{\text{O}}_{\text{2}}}$
(D) ${{\text{O}}_{\text{2}}}{\text{ + Ar}}$

Hint: Underwater diving is used to refer to the descending below the surface of the water to interact with the environment. Deep sea diving is underwater diving with surface supplied equipment. It is a very risky human activity as the diver is exposed to a wide range of hazards, such as breath-hold endurance limitations. Generally, heliox is used as a medical treatment for patients with difficulty in breathing.

Complete step by step answer:
(i) There are many constraints on underwater diving. The immersion of the human body in water affects the breathing, circulatory system, renal system and fluid balance. This is because the external hydrostatic pressure of water gives support against the internal blood pressure. As a result, blood shift occurs from the extravascular tissues of the limbs to the chest cavity.
(ii) The diving reflex results in slowed heartbeat which reduces the overall cardiac output. The decrease in lung volume takes place which in turn increases the resistance to airflow in the airways.
(iii) The reduction in internal oxygen supply underwater will increase the urge to breathe caused by carbon dioxide buildup in the blood and the diver may get drowned. That is why in deep sea diving, the divers are provided with surface supplied equipment.
(iv) Breathing gas mixture is supplied through a full face mask. The gas mixture usually supplied is heliox which is a breathing gas mixture of helium and oxygen. The proportion of oxygen in heliox mixture depends upon the maximum depth of the dive plan and it is typically ten percent. This arrangement allows the diver to breathe underwater.

So, the correct option is B.

Note: Sound travels faster in heliox than in air and so the voices of the divers are raised which in turn makes the diver’s speech very high-pitched and difficult to understand. A less expensive alternative to heliox for deep sea diving is trimix, which is a breathing gas mixture consisting of oxygen, helium and nitrogen.