Exam 220-1202 Topic 1 Question 96 Discussion

"No OS found" commonly indicates the firmware can't locate boot instructions for the installed OS. If the drive isn't obviously failing (no unusual noises) and BIOS/UEFI settings are reasonable, a classic cause is a corrupt Master Boot Record (MBR) (on MBR-partitioned disks). Mike Meyers' Lab Manual explains that when BIOS can't find an OS, causes may include configuration issues or drive problems, but also states: "If the correct drive is already first in the boot sequence, the master boot record... may be corrupt and need to be rebuilt." The Lab Manual also explains what the MBR does: the first sector contains boot code that informs the system about installed operating systems, and "without this bit of code, your OS will never load." The All-in-One guide describes the same: BIOS looks at the first sector for instructions, and without the MBR code, the OS won't load.
NTFS corruption wouldn't typically present as "no OS found" at the firmware stage, and TPM issues usually create different security/boot errors. Therefore, MBR (D) is most likely.