Final answer:
Controlled rate freezing is conducted with the addition of cryoprotectant and freezing at a predetermined rate per minute.
Step-by-step explanation:
The correct answer is b. The addition of cryoprotectant and freezing at a predetermined rate per minute.
Controlled rate freezing involves adding a cryoprotectant (such as glycerol or ethylene glycol) to the biological material to prevent ice crystal formation, which can damage the cells. The freezing process is then conducted at a predetermined rate per minute to ensure slow and controlled cooling. This helps to maintain the integrity of the cells during freezing and thawing.