Step 1: Picture a lake changing over centuries.
Left undisturbed, a lake slowly fills with sediment and nutrients from its surroundings and gradually shifts from clear and nutrient poor to rich in plant growth.
Step 2: Name the process.
This nutrient driven ageing of a water body is called eutrophication, whether it happens naturally over a long time or is sped up by human sources like fertiliser runoff and sewage.
Step 3: Rule out the other terms.
Biomagnification concerns toxin build up along a food chain, El nino is a climate and ocean current pattern, and algal blooms are just one visible symptom of eutrophication rather than the whole process.
Final answer: Option 1, eutrophication.