Option 1 -> Starts with "population as well as..." which cannot begin a sentence meaningfully.
Option 2 -> Begins with "role in scavenging..." without establishing subject or context first.
Option 3 -> Creates a coherent sentence: "According to environmentalists, the turtle plays a significant role in scavenging dead organic materials and diseased fish in the river, thereby controlling the fish population as well as aquatic plants and weeds."
Option 4 -> Starts with "fish in the river..." which lacks proper sentence structure and subject.
Hence, Option 3: (C), (A), (D), (B) -> The sequence correctly begins with the introductory phrase establishing the source (environmentalists), introduces the subject (turtle) and its role, then logically connects what it scavenges (diseased fish) to the ecological benefit (controlling fish population and aquatic plants/weeds) -> correct