The best detail supporting the significance of the nursery to the children is that it is described as 'their mother and father,' indicating the children's deep emotional attachment to it.
The detail from the text that best supports the answer to Part A, which pertains to the significance and impact of the nursery in the children's lives, is option B: "This room is their mother and father, far more important in their lives than their real parents." (Paragraph 198). This statement encapsulates the crux of the children's attachment and the room’s influence on them. It highlights the emotional bond and dependence the children have developed with the nursery, underscoring its significance as a parental figure that has supplanted their biological parents in terms of importance.