The correct answer would be D: succession has reached a climax.
In ecology, a community is said to be in equilibrium when its species composition and population sizes are stable over time. This is typically characteristic of a climax community, which has reached a state of balance where the ecosystem can sustain itself and maintain population levels without external disturbances.
Here's a brief explanation of the other options:
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A: pioneer organisms are increasing - This would indicate that the community is still in the early stages of ecological succession, not in equilibrium.
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B: immigration is occurring rapidly - Rapid immigration can result in fluctuations in population sizes, indicating that the community is not yet in equilibrium.
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C: the pyramid of energy has been reversed - A reversed energy pyramid is not a typical scenario in a stable ecosystem and implies an unbalanced or unhealthy community.
Therefore, option D is the most appropriate choice.