Final answer:
Elie realized that Rabbi Elahou's son deliberately abandoned his father during the death marches to increase his own chances of survival.
Step-by-step explanation:
The horrible realization that Elie came to concerning Rabbi Elahou and his son was b) Rabbi Elahou's son abandoned him. In the midst of the death marches from the concentration camps, Rabbi Elahou's son had noticed that his father was slowing him down. Seizing an opportunity during the chaos, he had increased his pace to move ahead, effectively abandoning his father and thereby ensuring his own survival, though it was a move made out of both desperation and survival instinct.