Final answer:
The benefit of grouping indicated by the graph is option B: 'dilution,' which implies that individual risk of predation is reduced when part of a larger group, as seen in schools of fish.
Step-by-step explanation:
The information in the graph that shows the number of predatory attacks per individual in water skaters groups of different sizes suggests that one benefit of grouping for prey animals is dilution. This concept implies that by being a part of a larger group, each individual has a lower chance of being preyed upon because there are more potential targets for the predator, thus diluting the risk to any single individual.
This is also related to the idea that prey animals in groups can protect one another from predators, as individual prey are less likely to be picked out by a predator. Schools of fish provide a good example of this, as they swim in a coordinated pattern which can rapidly move away from predators without chaotic movement that might lead to easier predation.