The following error is displayed on a user's computer screen:No operating system foundWhich of the...

The first step is to disconnect external storage (Option A). Sometimes, the system may be attempting to boot from an external drive or USB device instead of the internal hard drive. By removing the external storage, the system will attempt to boot from the correct drive.

Flashing the BIOS (Option B) is more complex and typically unnecessary for this issue.

Replacing the SATA cable (Option C) may help if there's a hardware issue, but it's not the first troubleshooting step.

Turning on the device in safe mode (Option D) would not work if no operating system is detected.

CompTIA A+ Core 2 References:

5.1 - Apply troubleshooting methodologies, including steps for resolving boot issues​​.