The given answer "kidney" is correct because the other terms mentioned in the question - nephro, pelvis, urethra, and bladder - are all related to the urinary system and are parts of the kidney. The kidney is an essential organ responsible for filtering waste products from the blood and producing urine. It is also connected to the other mentioned terms, such as the renal pelvis (which collects urine from the kidney), the urethra (which carries urine from the bladder to outside the body), and the bladder (which stores urine before it is released). Therefore, kidney is the correct answer.