The best place to store your client files is on your computer. The Option C is correct.
Storing client files on your computer is the most secure and convenient option when you are working on-site at your client's office. This ensures that your files are readily accessible without relying on an internet connection or external devices.
This also reduces the risk of data loss or security breaches that can occur when using external storage devices like flash drives or hard drives, which can be misplaced or compromised.
But it is important to maintain strong security measures on your computer such as encryption and regular backups, to protect sensitive client data.