Final answer:
External Hard Drive for storing videos, Cloud Storage for backups, and USB Flash Drive for file transfer.
Step-by-step explanation:
For storing a 30-minute video, the best storage device would be an External Hard Drive. External hard drives have large storage capacities and can hold large files such as videos.
For making a backup of accounting records each week, Cloud Storage would be the most suitable. Cloud storage provides remote backup and allows easy access to files from any device.
To transfer a 200 kilobyte file from one computer to another, the best storage device would be a USB Flash Drive. USB flash drives are portable and can be easily connected to different computers to transfer files.