A different way in is to isolate exactly what work the phrase "when they had not eaten for three days" is doing in the sentence, since a quote like this usually turns on one sharp detail.
That detail sits right next to "talking about books and an education", so the sentence is built to make readers notice the gap between severe hunger and a continued interest in learning. The passage is not arguing that education requires comfort, and it is not arguing that poor people should avoid books, since both of those readings contradict the admiring tone toward the women.
What the detail does support is the simple, direct claim that people can and do care about books and education even while going hungry, which is exactly the idea in option B.
So the correct answer is B.
A third way to check is to ask what the author is trying to persuade the reader of, since a Nobel lecture like this is built to make an argument, not just describe a scene.
Lessing places two images side by side on purpose: comfortable people "stuffed full of food" and "stifling" in excess, against hungry women still discussing books. The persuasive point only works if the reader accepts that the hungry women's interest in books is real and admirable despite their hunger, since that contrast is what "may yet define us" refers to.
Options that claim poor people should avoid books, or that comfort is a requirement for valuing education, both undercut the very contrast the author is building, while the claim that hungry people can still care about books and education is exactly what makes the author's persuasive point land.
So the correct answer is B.

