The correct option is B liver
Synthesis of urea takes place in the liver. The nitrogen in the amino acid is converted to ammonia, and this is further converted into urea. Urea is less toxic to the body compared to ammonia. The reactions take place in a cyclic manner called urea cycle. The kidneys then filter out the urea from the circulation, sends it to the bladder along with water and excretes it in the form of urine.