Explanation:

Preserving germplasm (genetic resources like seeds, pollen, tissues, or cells) in a frozen state, typically at ultra-low temperatures (e.g., in liquid nitrogen at $-196^\circ\text{C}$), is precisely the definition of Cryopreservation. This method ensures long-term viability by halting metabolic processes. Option B, In situ preservation, refers to conserving species in their natural habitats or ecosystems. Option C, Cold storage, is a general term for storing items at low temperatures, but it does not specifically imply the frozen state required for long-term germplasm viability and is less precise than cryopreservation. Option D, Vernalisation, is a process where plants are exposed to a period of cold temperature to induce flowering, which is a physiological response, not a preservation technique. Therefore, cryopreservation is the correct term.