Final answer:
The term that best describes a close association between organisms of different species in grassland is symbiosis, which includes relationships like mutualism where both species benefit.
Step-by-step explanation:
The relationship in a grassland that best describes a close association between organisms of different species where at least one organism benefits is called symbiosis. In symbiosis, there are three types of relationships: mutualism, where both species benefit; commensalism, where only one species benefits without harming the other; and parasitism, where one species benefits at the expense of the other. An example of mutualism in a grassland might be the relationship between certain species of ants and aphids, where ants protect the aphids from predators and receive a sweet substance called honeydew in return.