Commensalism is a type of symbiotic relationship between two organisms where one organism benefits while the other organism is neither helped nor harmed. This means that the option that best describes commensalism is "one of the organisms gains while the other neither gains nor loses."
In commensalism, the organism that benefits is called the commensal, while the organism that is neither helped nor harmed is called the host. The commensal benefits from the association by gaining resources such as shelter, food, or a place to live, while the host is not affected either positively or negatively.
Therefore, commensalism is a relationship where one organism gains while the other is unaffected, making it different from other types of symbiotic relationships such as mutualism and parasitism where both organisms can either benefit or one organism gains while the other loses, respectively.