Elasmobranchii and Adult Amphibians live in sea water which is hypertonic to them. They need to conserve water thus excrete Urea. The Urea also acts as osmoregulator for Adult Amphibians and Elasmobranchii. To maintain their water balance, Elasmobranchii and Adult Amphibians drink large quantities of seawater, retaining most of the water and excreting the salt. So, it is very main excretory product for them (Elasmobranchii and Adult Amphibians).

So, Option b is correct.

Urea as the main excretory product in elasmobranchii and adult amphibians

Introduction
- Urea is a nitrogenous waste product that is formed in the liver of animals as a result of protein metabolism.
- It is a highly soluble compound that can be excreted in the urine.
- Different groups of animals excrete different types of nitrogenous wastes depending on their physiological adaptations and habitats.

Excretion in Protozoans, Amphibians, and Reptiles
- Protozoans, such as amoeba and paramecium, are simple organisms that live in water.
- They excrete their nitrogenous waste mainly in the form of ammonia (NH3) which is highly toxic.
- Ammonia is produced in the cytoplasm of these organisms and diffuses out into the surrounding water.
- Amphibians and reptiles are also primarily aquatic or semi-aquatic animals.
- They excrete their nitrogenous waste mainly in the form of urea which requires a large amount of water for its excretion.
- Urea is less toxic than ammonia but still requires a significant amount of water for its elimination.
- Therefore, option 'A' (protozoans, amphibians, and reptiles) is incorrect as urea is not the main excretory product in protozoans.

Excretion in Elasmobranchii and Adult Amphibians
- Elasmobranchii is a group of cartilaginous fish that includes sharks, rays, and skates.
- These organisms are adapted to living in marine environments.
- They excrete their nitrogenous waste mainly in the form of urea, which helps them maintain osmotic balance in the high salinity of seawater.
- Urea is less toxic than ammonia and can be retained in their body fluids, reducing water loss.
- Adult amphibians, such as frogs and toads, also excrete their nitrogenous waste mainly in the form of urea.
- They have permeable skin that allows them to absorb water from their surroundings, reducing the need for large amounts of water for excretion.
- Therefore, option 'B' (elasmobranchii, adult amphibians) is correct as urea is the main excretory product in these organisms.

Excretion in Reptiles, Birds, and Mammals
- Reptiles, birds, and mammals are terrestrial animals that have evolved different adaptations for water conservation.
- They excrete their nitrogenous waste mainly in the form of uric acid, which is a highly insoluble compound.
- Uric acid can be excreted as a semi-solid paste, minimizing water loss.
- Therefore, option 'C' (reptiles, birds, and mammals) is incorrect as urea is not the main excretory product in these organisms.

Excretion in Insects, Birds, and Fishes
- Insects, such as cockroaches and grasshoppers, excrete their nitrogenous waste mainly in the form of uric acid.
- Birds, like reptiles, also excrete their nitrogenous waste mainly in the form of uric acid.
- Fishes, including bony fish, excrete their nitrogenous waste mainly in the form of ammonia.
- Therefore, option 'D' (in