Final answer:
The correct statement about the excretory system is that the kidneys contain nephrons, which are the filtration units where urine is formed. Villi are not found in kidneys, the bladder does not have two urethras, and there is only one bladder connected to the ureters.
Step-by-step explanation:
Among the options provided, the true statement about the excretory system is a. the kidneys contain nephrons. The nephrons are the functional units of the kidneys where blood filtration occurs. Each kidney contains more than a million nephrons which are involved in the process of removing wastes, concentrating them, and forming urine.
Option b, the kidneys contain villi, is incorrect because villi are found in the small intestine where they increase the surface area for absorption, not in the kidneys. Option c, the bladder has two urethras, is false because there is only one urethra that drains urine from the bladder. Lastly, option d, each bladder connects to a ureter, is incorrect since there is only one urinary bladder that is connected to two ureters, each coming from one of the kidneys.