The correct choice is (a) Hot air sterilization
The best I can explain: Gauze and cotton are cloth materials which have their own pores. Using moist sterilization will cause them to become wet and they may not dry out. Chemical sterilization also poses the same problem, the chemicals may stay back and adhere to the gauze and cotton. For sterilization under pressure, again moisture is required. Thus, hot air sterilization is used. Hot air at high temperature and pressure is blown and the gauze and cotton are sterilized.