Answer:
A. At least once a week
Explanation:
Based on the information provided within the question it can be said that in this scenario you should update the software at least once a week. This is because antivirus protection software's have a definition file which holds various information on the new types of threats that are being created every day. Therefore updating this file is crucial for the software to be able to do it's job effectively. Therefore the best option would be to update it as often as possible.
