What feature makes the root systems of mangroves in sandy shores unstable?

The root systems of mangroves in sandy shores are considered unstable largely due to the movement of fine mineral particles by wind and tides. This ongoing movement can cause the substrate to shift and erode, making it difficult for mangroves to establish strong anchorage. Mangrove roots are designed to stabilize the trees in soft sediment; however, in sandy environments, when fine particles are displaced, the roots might not be able to secure themselves effectively. The continuous disturbance from environmental forces can lead to a lack of structural integrity for the root systems, ultimately affecting the health and growth of the mangrove trees.

Other options presented do not directly influence the stability of the root systems in the same way. For instance, large rocks can actually impede root growth rather than making them unstable, while a lack of sunlight largely affects the plant's ability to photosynthesize rather than directly impacting the stability of the roots. Additionally, while water depth can impact the growth of mangroves, it is not the movement or presence of sediment that creates instability; instead, it is the dynamic nature of sandy environments that poses the most significant challenge to their root systems.