Final answer:
The smallest ecological entity is an individual, which refers to a single organism. It forms the basis of the ecological hierarchy, with populations, communities, and ecosystems as progressively more complex levels.
Step-by-step explanation:
The smallest ecological entity is D) individual. In the hierarchy of biological organization, the individual is the most basic unit. An individual refers to a single organism which can be distinguished from other entities and is the simplest level of organization in ecology. As we move up the hierarchy, individuals group together to form populations, which are all the members of a species living within a specific area.
Multiple populations interact within a community, encompassing all biotic, or living, entities in an area. Further expanding the complexity, a community along with its abiotic, or non-living, environmental factors such as water, sunlight, and soil nutrients, form an ecosystem. These levels are studied within various branches of ecology such as organismal ecology, population ecology, community ecology, and ecosystem ecology.
The hierarchy can be observed in a typical forest ecosystem where individual trees, insects, and animals represent the basic units. The pine trees in the forest make up a population, and when you consider all the different populations of trees, flowers, insects, and other organisms together, they form the forest's community. Finally, the forest's community, in conjunction with the soil, water, and other nonliving components, represents an ecosystem.