Helium is mixed with oxygen for the respiration by deep sea divers, because

• Deep inside water the pressure is very high and hence divers tends to produce bends in their nerves which are extremely risky and life threatening so in order to solve the problem the mixture in cylinder contains helium(majorly) and nitrogen(both are insoluble in blood) which along with oxygen maintains air pressure inside nerves or body and prevents bending. 

Hence option A is the correct answer.