Which of the following defines symbiosis?

Symbiosis is defined as a relationship between two different species of organisms that live in close association with each other. Of the options given, the one that defines symbiosis is "both organisms benefit mutually by living together." This refers to mutualism, which is a type of symbiotic relationship in which both organisms benefit from their association with each other. In mutualism, both organisms provide some benefit to the other, such as food, protection, or transportation. This type of symbiotic relationship is common in nature, such as the relationship between pollinators and flowers or the relationship between certain species of ants and aphids. The other options given do not define symbiosis. Parasites feed on other organisms, often harming them in the process, and saprophytes feed on dead organisms, which is not a symbiotic relationship. Organisms that serve as hosts to parasites are not benefiting from the relationship, and the relationship is not mutualistic. Therefore, symbiosis refers to a close association between two different species of organisms in which both organisms benefit from the relationship, such as in the case of mutualism.