It's uric acid as it can be removed with a minimum loss of water....just refer ncert .

The least toxic nitrogenous waste is Uric acid.
Explanation:
Uric acid is considered the least toxic nitrogenous waste among the options given. Here's why:
1. Ammonia:
- Ammonia is highly toxic to the body and requires a lot of water to dilute it.
- It is mainly excreted by aquatic organisms, such as fish, in the form of ammonia.
- High levels of ammonia in the body can lead to problems like ammonia poisoning.
2. Urea:
- Urea is the primary nitrogenous waste product in mammals, including humans.
- It is formed in the liver by the breakdown of amino acids and is less toxic than ammonia.
- Urea requires less water to dilute it, making it the main nitrogenous waste in terrestrial animals.
- Urea is excreted by the kidneys in urine.
3. Uric Acid:
- Uric acid is the least toxic nitrogenous waste among the given options.
- It is the primary nitrogenous waste in reptiles, birds, and insects.
- Uric acid is relatively insoluble in water, allowing it to be excreted as a paste or solid, reducing water loss.
- This is an advantage for organisms living in arid environments or those with limited access to water.
In summary, while urea is the primary nitrogenous waste product in mammals, including humans, uric acid is the least toxic nitrogenous waste among the options given. It is excreted by reptiles, birds, and insects, allowing for conservation of water in arid environments.