Option 1: (C), (A), (B), (D) -> Creates a logical flow: subject introduction → description → problem → consequence → complete relative clause.
Option 2: (C), (A), (D), (B) -> Breaks the sentence structure by separating "people who" from "rely on it" and leaves the sentence incomplete.
Option 3: (B), (A), (D), (C) -> Starting with "rising seas" makes no grammatical sense as the sentence opener.
Option 4: (C), (B), (D), (A) -> Disrupts the natural progression and creates an awkward sentence structure.
Hence, Option 1: (C), (A), (B), (D) -> The correct sentence reads: "The Sundarbans, the world's largest mangrove forest and home to a wide variety of species is shrinking due to deforestation and rising seas, posing a threat to the 7.5 million people who rely on it as a natural barrier against tides and cyclones." This arrangement maintains proper grammatical structure, logical flow, and completes the relative clause appropriately. -> correct