Answer: The best replacement for the sentence "One day, she teaches her own children and grandchildren this precious recipe!" is "One day, she would teach her own children and grandchildren this precious recipe".
Explanation: The best replacement for the sentence provided is "One day, she would teach her own children and grandchildren this precious recipe" because the speaker is looking forward in time or referring to something that she will do in the future but from a specific point in the past and "would" is a modal verb that can be used to express this idea. In other words, "would", together with a bare infinitive such as "teach", can be used to speak about the future in the past. In contrast, "had taught", "is teaching" and "teaches" cannot be used in this way.
