Ammonia is the main nitrogenous excretory material in Tadpoles.

Ammonia as a nitrogenous waste:
- Nitrogenous waste products are produced as a result of protein and nucleic acid metabolism in organisms. These waste products need to be eliminated from the body as they can be toxic if accumulated.
- Different animals have different mechanisms to convert nitrogenous waste into less toxic forms. One such form is ammonia (NH3), which is highly toxic and needs to be eliminated quickly.
- Ammonia is a small, water-soluble molecule and can be easily excreted by aquatic organisms.

Tadpoles and ammonia excretion:
- Tadpoles are the larval stage of amphibians, such as frogs and toads. They live in water and respire through gills.
- Tadpoles excrete ammonia as their main nitrogenous waste product. This is because they live in an aquatic environment and can easily eliminate ammonia through their gills into the surrounding water.
- Ammonia diffuses out of the tadpole's body and into the water, where it gets diluted and is less toxic.
- This process of ammonia excretion in tadpoles is called ammonotelism.

Advantages of ammonia excretion in tadpoles:
- Ammonia is highly soluble in water, allowing for its easy elimination through the gills.
- Tadpoles have a large surface area for gas exchange through their gills, which facilitates the elimination of ammonia.
- Ammonia excretion is energetically inexpensive for tadpoles as it does not require much energy for conversion or storage.

Other animals and nitrogenous waste excretion:
- Other animals, such as reptiles, birds, and mammals, have different mechanisms for nitrogenous waste excretion.
- Reptiles (option D in the question) excrete uric acid as their main nitrogenous waste. Uric acid is less toxic and less water-soluble than ammonia and can be excreted as a paste or solid.
- Birds (option B in the question) also excrete uric acid as their main nitrogenous waste. This adaptation helps them conserve water as uric acid requires less water for excretion.
- Mammals, including humans, excrete urea as their main nitrogenous waste. Urea is less toxic than ammonia and can be excreted in a more concentrated form, requiring less water loss.

In conclusion, tadpoles excrete ammonia as their main nitrogenous waste because they live in an aquatic environment and can easily eliminate it through their gills. This adaptation allows for efficient excretion of waste without excessive water loss.