Final answer:
The adaptation that would best help tall trees to survive a flood would be the development of air spaces in the trunk, similar to the pneumatorphores found in mangroves and cypress trees, which allow for air exchange and oxygen supply to the roots in waterlogged conditions.
Step-by-step explanation:
When considering adaptations that would most help tall trees survive a sudden two-meter rise in sea level leading to flooding, the most beneficial adaptation would be the development of air spaces in trunks, facilitating air transfer to the roots. This adaptation is observed in some species of mangroves and cypress trees, which possess pneumatorphores—upward-growing roots with gas exchange capabilities. These pneumatophores contain pores and pockets of tissue specialized for taking in air, allowing the plants to obtain oxygen even when submerged in water with little availability of it in the soil.
In contrast, the other options listed would not be as effective. A broad, shallow root system would likely become waterlogged and fail to provide sufficient support in the soft, flooded soil. Low-density wood might help the tree to float initially, but it would not support long-term survival and growth. Lastly, low hanging branches would be more susceptible to water damage and could hinder the tree's ability to photosynthesize if submerged.
Therefore, the adaptation that would most aid tall trees in such a flooding scenario is air spaces in the trunk, similar to the aerenchyma found in aquatic plants like wild rice, allowing oxygen to diffuse down to the roots and ensure survival despite the inundated environment.