Final answer:
In a rocky intertidal zone, mussels usually dominate the middle area as they can firmly attach to the rocks and resist the wave action.
Step-by-step explanation:
The middle intertidal zone of a rocky shoreline is often dominated by mussels (B). This is due to the strong attachment they have to the rocks with their byssal threads, which allows them to resist the pounding waves and avoid being washed away. Since barnacles also secrete a cement-like substance that anchors them firmly to the rocks, they are a close competitor to mussels in this zone, although mussels are typically more dominant.