The correct answer is option 1, which states that the optimum dissolved oxygen level required for the survival of aquatic organisms is 4-6 mg/litre. Dissolved oxygen is essential for the respiration and metabolism of aquatic organisms. Option 2, which suggests a range of 2-4 mg/litre, would be insufficient for most aquatic organisms to survive. Option 3, with a range of 8-10 mg/litre, is higher than what is typically required and could make it difficult for organisms to acquire enough oxygen. Option 4, with a range of 12-16 mg/litre, is even higher and unnecessary for the survival of most aquatic organisms. It`s important to note that different species of aquatic organisms have varying oxygen requirements, but the general consensus is that a dissolved oxygen level of 4-6 mg/litre is optimal for survival.