The temperature inside a freezer unit used to store vaccines must be kept at:

The correct answer is based on the specific temperature requirements for the proper storage of vaccines that need to be kept in a freezer environment. Vaccines such as some live attenuated vaccines and certain inactivated vaccines require storage in a deep freeze to maintain their efficacy and stability.

A temperature range of -50 degrees Celsius to -15 degrees Celsius ensures that the vaccines are stored at sufficiently low temperatures to prevent degradation. This range aligns with the recommendations from health organizations for vaccines that require freezing. Keeping vaccines within this temperature threshold helps preserve their potency and ensures they remain effective when administered to patients.

Maintaining temperatures outside of this range could lead to improper storage, which might compromise the vaccines' effectiveness, potentially leading to ineffective immunization and public health risks. Thus, the correct temperature range is critical for the safe and effective storage of certain vaccines.