I believe the correct answer is D. to immediately show the difficult relationship that Negi once had with her mother.
Usually, when kids get hurt somehow, parents will rush towards them to help them, hug them, and stop them from crying. But as you can see here, in this text that isn't the case. Negi burned her fingers, and was probably scared and in pain, yet her mother just said that it will teach her not to do the same thing again. It shows us a strained relationship between the mother and her daughter that would exist for a very long time in Negi's life.