The red color of the Red Sea is due to the presence of Option B, algae. Trichodesmium erythraeum, a type of cyanobacteria, is primarily responsible for this phenomenon. These organisms contain pigments such as phycoerythrin, which impart a reddish hue to the water when they bloom in large quantities. This occurrence is often referred to as a “red tide” or “sea sawdust” and can result in significant changes to the marine ecosystem. While other factors such as mineral sediments and dissolved organic matter can contribute to the sea’s coloration, the predominant cause of the Red Sea’s red coloration is the proliferation of these algae. Moss (Option A), fungus (Option C), and bacteria (Option D) are not typically associated with causing the red coloration observed in the Red Sea.