The best response that captures the cost and benefit of farming in the situation described is: "Successful harvest not guaranteed due to weather, but more people can cooperate to build better shelters."
This response highlights the uncertainty associated with farming, particularly due to environmental factors like weather, which aligns with the family's experience of flooding and crop loss. It also emphasizes the benefit of cooperation within the multi-generational family, allowing them to create better solutions to their challenges, such as improving shelter.