Obligate symbiosis is characterized by:

• A. Organisms that choose to live together
• B. Organisms that can survive independently
• C. Organisms that cannot survive without each other
• D. Organisms that compete for resources

Answer: (C)

Obligate symbiosis is characterized by the relationship between two organisms that are so interdependent that they cannot survive without each other. This type of symbiosis often involves two species that have co-evolved to the extent that their survival, reproduction, or both are entirely reliant on their partnership. Common examples include certain species of fungi and algae that form lichens or specific types of bacteria that live within the gut of animals, providing essential nutrients.

In this context, the other choices describe characteristics that do not fit the definition of obligate symbiosis. For instance, organisms that choose to live together or those that can survive independently each describe more voluntary or temporary relationships. Similarly, competition for resources indicates a struggle between organisms for limited supplies, which is in contrast to the mutual dependency that characterizes obligate symbiosis.