A server failed Thursday evening before the nightly backup. The backup scheme in place was full backups every Sunday, incremental backups on Monday, Wednesday, and Friday, and differential backups…

One backup set (only the Sunday full backup) would not restore any data changed after Sunday.

Two backup sets would not be sufficient to restore all data up to the end of Wednesday, as it would either miss Wednesday's incremental changes or Tuesday's differential changes if an incremental chain is followed.

To restore the server to its state at the end of Wednesday, you need the last full backup (Sunday), followed by the Monday's incremental backup (changes since Sunday), and then Wednesday's incremental backup (changes since Monday's incremental). This sequence reconstructs the data up to the point of failure.

Four backup sets would be more than the minimum required; the correct combination for this scenario can be achieved with three sets.