Final answer:
The purpose of knocking on Elie's family's window the night of the council's decisions was to tell them that they have been selected for deportation.
Step-by-step explanation:
The purpose of knocking on Elie's family's window the night of the council's decisions was B. To tell them that they have been selected for deportation. This event likely took place during the time when ghettos were being liquidated and Jews were being sent to concentration camps.