Cryo-preservation is a method associated with:

✅Explanation:
Cryopreservation is a process where biological materials, such as cells, tissues, or organs, are preserved by cooling them to very low temperatures, typically using liquid nitrogen. Liquid nitrogen has a boiling point of -196°C (-321°F), which is low enough to essentially stop all biological activity, including the processes that lead to cell death and degradation.

• Why Liquid Nitrogen is Used in Cryopreservation:
Extremely Low Temperature: Liquid nitrogen provides the extremely low temperatures needed to effectively preserve biological materials for extended periods.
Inert: Liquid nitrogen is inert and does not react chemically with the preserved materials, ensuring their integrity.
Safe: While handling liquid nitrogen requires caution, it is a safe and reliable cryoprotectant when used properly.

📌Other Options Explanations:
(b) Freezing: While freezing is part of cryo-preservation, it specifically refers to ultra-low temperature freezing using liquid nitrogen.
(c) Chemical: Chemicals, known as cryoprotectants, are used in conjunction with freezing to prevent ice crystal formation and damage to cell.
(d) Drying: Drying, or desiccation, is a preservation method that removes water from biological materials.

🔑Key Points:
Cryopreservation is the method of keeping living cells, tissues, eggs, sperm, and other biological samples in a deep freeze at subzero temperatures for storage or preservation.
Cryopreservation is also a process of Ex situ conservation.
The sample is commonly kept at liquid N2 gas and its temperature maintains at −196º C.
It is an effective way to transport biological samples over long distances, and store samples for prolonged periods of time.
The threatened species can be preserved in viable and fertile conditions for long periods using cryopreservation techniques, eggs can be fertilized In vitro, and plants can be propagated using tissue culture.
The steps involved in preserving the biological samples are,
Harvesting or Selection of material
Addition of cryo-protectant
Freezing
Stored in liquid nitrogen gas
Thawing
In cryopreservation, cryo-protectants are used to reduce the freezing point of the medium, dehydrate the preservable substances, and slow the cooling rate, which reduces the risk of crystallization.
Examples of cryo-protectants are glycerol, DMS, FBS, trehalose, glycols, etc.
Glycerol and DMS (Dimethyl sulfoxide) are used to dehydrate the biological materials during cryopreservation.
Glycerol and DMS prevent dehydration and intracellular ice crystals formation, which can result in cell death and destruction of cell organelles during the freezing process. 
DMS is broadly used for the protection of most cells and tissues and glycerol is commonly used for the cryoprotection of RBCs.