Final answer:
The most useful trait for growing corn in areas with short summers is a fast growth rate, as it allows the plant to complete its lifecycle within a limited timeframe.
Step-by-step explanation:
The trait that would be most useful for growing corn in areas with short summers is fast growth rate. A fast growth rate would allow corn to complete its lifecycle more quickly, which is crucial in regions where the growing season is limited by the climate. Contrary to traits like deep roots (which are essential for survival in arid conditions with sparse water supplies), or large seed size and insect resistance, the fast growth rate ensures that the plant can germinate, grow, flower, and produce seeds before the end of the short summer season. While insect resistance is important, it is secondary to the need for rapid development in short summer condition. Additionally, while a large seed size might contribute to the initial stages of growth, it is not as critical as a quick lifecycle for the overall yield before the onset of colder temperatures.