Final answer:
The sentence should use the preposition 'of' to correctly express the relationship, making it 'I made the most of the years when I had the undivided attention of my adult relatives.'
Step-by-step explanation:
The sentence in question, "As the firstborn grandchild, I made the most to the years when I had the undivided attention of my adult relatives?" requires the correct preposition to make it grammatically correct. The correct preposition to use in this context is 'of', which indicates a relationship involving a part of something or a belonging. Therefore, the sentence should read, "As the firstborn grandchild, I made the most of the years when I had the undivided attention of my adult relatives." The correct answer is option 3) of.