Hint: During rancidification, oils and fats undergo complete oxidation or hydrolysis. We can initiate rancidity of oils and fats either by exposure to sunlight or moisture or by bacterial action.
The gases oxygen and carbon dioxide do not help in preventing the rancidity of oil. Hence, the following options are incorrect options.
A) Carbon dioxide or oxygen
B) Nitrogen or oxygen
C) Carbon dioxide or helium
When oil comes in contact with air or atmospheric oxygen, it becomes rancid. Rancidity is oxidative degradation of an oil. We can avoid the rancidity of oil, if you can avoid contact between oil and air. For this purpose, we can cover the surface of oil with helium or nitrogen.
Thus, gases such as helium or nitrogen prevent the contact of air (or atmospheric oxygen) with oil. In this way, we can prevent oil from becoming rancid for a long period of time.
Helium or nitrogen gases can be used for storage of a fresh sample of an oil for a long time.
Hence, the correct option is the option (D).
Note: Due to the rancidification process of oils and fats, you will observe change in colour, smell and test. For example, if you expose a packet of chips to air, we will observe a change in smell and taste.