Final answer:
Mistress Owens wants to adopt the mysterious baby both because she never had a child of her own and because the boy's mother's ghost asks them to keep him safe.
Step-by-step explanation:
The question appears to refer to the novel The Graveyard Book by Neil Gaiman, in which a baby boy who wanders into a graveyard is adopted by the ghosts of Mistress and Mr. Owens. The reasons Mistress Owens wants to adopt the mysterious baby are both A and B: A. Because she and Mr. Owens never had a child while they were alive, and B. Because the ghost of the boy's mother begs them to keep her son safe. Their desire for a child and the plea from the boy's mother are both compelling motives for Mistress Owens' decision to adopt the boy.