A

[spoiler alert:] I see someone in the comments has asked why they didn’t just take the father with themselves; that would apparently solve the main problem. Let’s suppose that there’s no complication in taking the old man with themselves. The important point somehow mentioned or implied in a number of occasions is that Nader’s problem with leaving (or his motivation for staying) is seemingly beyond his father. After all, we find out from the last scene that the father is already dead, although Nader hasn’t changed his decision. Moreover, there’s a dialogue earlier in the movie in which he tells his wife that he wants their daughter to stay and learn to fight (for her rights, a better society, etc.). All these imply that his reason for staying most likely involves a greater responsibility and compassion; the father might as well work as a symbol for this greater responsibility.