The correct answer is option 'B', the urinary bladder opens into the urethra in mammals. Let's understand why this is the correct answer.

The urinary system in mammals is responsible for the production, storage, and elimination of urine. It consists of several organs, including the kidneys, ureters, urinary bladder, and urethra. The urinary bladder is a hollow muscular organ that stores urine temporarily until it is expelled from the body through the process of urination.

Here is a detailed explanation of each option and why option 'B' is the correct answer:

a) Uterus: The uterus is a part of the female reproductive system and is not directly involved in the excretion of urine. It is responsible for housing and nourishing a developing fetus during pregnancy.

b) Urethra: The urethra is a tube-like structure that connects the urinary bladder to the external environment. It serves as a passageway for urine to exit the body during urination. In males, the urethra also serves as a passage for semen during ejaculation.

c) Vestibule: The vestibule is a part of the female reproductive system and is located between the labia minora. It contains the openings of the urethra and the vagina. While the urethra is connected to the vestibule, it does not open directly into it.

d) Ureter: The ureter is a tube-like structure that connects each kidney to the urinary bladder. It carries urine from the kidneys to the bladder for storage. The ureters do not open into the bladder directly.

In conclusion, in mammals, the urinary bladder opens into the urethra. The urethra serves as a conduit for urine to exit the body during urination.