Final answer:
Grandma Cobb’s random visits to the villagers’ homes helps develop the idea of underserved rewards. So the correct answer is Option D.
Step-by-step explanation:
The detail that helps the author develop the idea of underserved rewards is Grandma Cobb’s random visits to the villagers’ homes. These visits suggest that Grandma Cobb is being rewarded with the villagers' attention and hospitality, even though they may not have much to offer. This highlights the contrast between the rewards Grandma Cobb receives and the lack of attention that the protagonist, Bernice, feels she receives when she is away from home.