Answer: B. red tide
Step-by-step explanation:
Red tide is a phenomena that is caused by algal bloom specifically due to the growth and abundance of unicellular algae called as dinoflagellates and diatoms. These organisms impart red color to the coastal waters. Such kind of algal blooms release toxins in the water. They deplete the concentration of oxygen in the water body.
The shellfish consume these small unicellular organisms as they filter large amount of water. The shellfish population accumulates large amount of toxins on consumption of these small organisms. The shellfish are harvested and consumed by the human population can make them sick and may lead to mortality of large population.