'o καθενας' here is also correct, indeed better. The given answers are roundabout.
"Everyone" translates to "καθένας" or "όλοι"?
Όλοι. Ο καθένας translates "each".
Is Όλα always wrong in this case... Even if it's short for Όλα (τα άτομα) χρειάζονται for instance?
Όλα without a clear reference refers to things, not living creatures, so it sounds very odd.
But "όλοι χρειάζονται αίμα" was also just rejected. Only the periphrastic "όλοι οι άνθρωποι..." can be accepted here?