Ureotelic mode of excretion is common to both humans and adult frogs.

Explanation:

Ureotelic mode of excretion refers to the excretion of nitrogenous waste in the form of urea. Urea is a compound that is formed in the liver by the breakdown of amino acids, and it is less toxic than other nitrogenous waste products such as ammonia and uric acid. Both humans and adult frogs excrete nitrogenous waste in the form of urea.

Humans:
- In humans, the kidneys are responsible for the excretion of nitrogenous waste.
- The kidneys filter blood and remove waste products, including urea, from the bloodstream.
- The urea is then excreted in the urine, which is stored in the bladder and eliminated from the body through the urethra.

Adult Frogs:
- Adult frogs excrete nitrogenous waste through their kidneys as well.
- They have specialized structures called nephrons in their kidneys, which filter waste products from the blood.
- The waste products, including urea, are then excreted in the urine.
- However, frogs also have an additional mode of excretion called cutaneous respiration, where they exchange gases and excrete some waste products through their skin.

Importance of Ureotelic mode of excretion:
- Ureotelic mode of excretion is important for maintaining the balance of nitrogenous waste in the body.
- Urea is less toxic than ammonia and uric acid, making it a safer compound for excretion.
- By excreting urea, humans and adult frogs can prevent the buildup of toxic waste products and maintain homeostasis.

Other options:
- Four-chambered heart: This characteristic is only present in humans and some other mammals, not in adult frogs. Adult frogs have a three-chambered heart.
- Internal fertilization: This characteristic is only present in humans and some other animals, not in adult frogs. Frogs have external fertilization, where the eggs are fertilized outside the female's body.
- Nucleated RBCs: This characteristic is present in frogs but not in humans. Adult frogs have nucleated red blood cells, while humans have anucleated red blood cells.