Based on the graph above, Option D which agrees that "Water has a high specific heat, which maintains relatively stable temperatures for aquatic organisms, most of whom are ectotherms" is correct as that's the major reason for the difference in the rate of temperature change between sand and water from the graph shown.
Firstly, ectotherms, for example, most aquatic organisms depend on their surroundings to help them maintain relatively stable temperatures of their bodies. On the other hand, endotherms for example humans, don't require the environment to keep their temperature stable because they have the capacity to generate most of the heat, they need internally.
Sand requires a low amount of energy for 1 gram of its mass to be raised by a temperature of 1°C. This makes sand have a relatively low specific heat. On the other hand, water is very resistant to changes in temperature making it have high specific heat.
From the graph, you would notice as the day gets hotter (moving from morning to afternoon), the temperature of the sand increases meaning the gets hot so fast. While the temperature of the water remains almost constant.
The high specific heat of water makes it a very good property for most aquatic organisms that are ectotherms to survive.
Option A (Sand has a low specific heat, which allows land-dwelling endotherms to regulate their temperature more easily) is not correct because though, Sand has low specific heat, it doesn't concern endotherms since they can regulate their body temperatures themselves.
Option B is not right too, Water doesn't have low specific heat, as I've explained above.
Option C is not correct too, as sand doesn't have high specific heat.
In summary, Based on the graph above, Water has a high specific heat, which maintains relatively stable temperatures for aquatic organisms, most of whom are ectotherms.
Learn more about the specific heat of water here: brainly.com/question/22113163