When a water body becomes extremely low in nutrient content, then it is called as.

The correct option is Oligotrophic.

Limnologists (scientists who study freshwater systems) classify water bodies based on their primary productivity and nutrient levels, particularly phosphorus and nitrogen. This classification system is called the trophic state index (TSI). Let's understand each of the options to see why "Oligotrophic" is the correct term for a water body with extremely low nutrient content:

1. Oligotrophic: The term comes from the Greek words oligo meaning "few" and trophos meaning "nourishing". Therefore, oligotrophic water bodies have very low nutrient concentrations. Because of the scarce nutrients, they support very little algal growth, resulting in highly clear, clean water with high levels of dissolved oxygen throughout the water column.

2. Mesotrophic: The prefix meso means "middle". Mesotrophic water bodies have a moderate level of nutrients and moderate productivity, representing a transition state between low and high nutrient levels.

3. Eutrophic: The prefix eu means "well" or "good". Eutrophic water bodies are rich in nutrients, leading to high biological productivity. This often results in frequent algal blooms, murky green water, and potential oxygen depletion (hypoxia) in deeper water layers.

4. Dystrophic: The prefix dys means "ill" or "bad". Dystrophic water bodies, typically bogs or peatlands, are characterized by highly acidic water rich in organic matter (humic substances) which gives the water a brown or tea-like color. These are not defined primarily by low nutrient status alone but rather by their organic acids and low pH.

Thus, a water body that has becomes extremely low in nutrient content is specifically referred to as Oligotrophic.