To solve this problem, we need to analyze each statement to determine which one is correct:$\newline \bullet$Option 1: ‘Eutrophication’ refers to increase in domestic sewage and waste water in lakes.$\newline$ - Eutrophication is actually the process where water bodies become enriched with nutrients, leading to excessive growth of algae and other aquatic plants.$\newline$ - It is not specifically about domestic sewage and waste water, but rather about nutrient enrichment.$\newline$ - Therefore, this statement is incorrect.$\newline \bullet$Option 2: Biomagnification refers to increase in concentration of the toxicant at successive trophic levels.$\newline$ - Biomagnification is the process where the concentration of a toxic substance increases in organisms at higher trophic levels in a food chain.$\newline$ - This is a correct definition of biomagnification.$\newline$ - Therefore, this statement is correct.$\newline \bullet$Option 3: Presence of large amount of nutrients in water restricts 'Algal Bloom'.$\newline$ - In reality, the presence of large amounts of nutrients in water promotes algal blooms, not restricts them.$\newline$ - Therefore, this statement is incorrect.$\newline \bullet$Option 4: Algal Bloom decreases fish mortality.$\newline$ - Algal blooms can lead to decreased oxygen levels in water, which can increase fish mortality.$\newline$ - Therefore, this statement is incorrect.$\newline$Based on the analysis, the correct option is Option 2, which accurately describes biomagnification.