Which of the following structures is a muscular sac at the bottom of the abdominal cavity?

The urinary bladder is a muscular sac located at the bottom of the abdominal cavity. It serves as a reservoir for urine, allowing the body to store it until the process of urination is initiated. The bladder's muscular walls enable it to expand as it fills with urine and contract to expel the urine during urination, demonstrating its functional significance in the urinary system.

The other structures mentioned do not serve the same purpose. The ureter is a duct that carries urine from the kidneys to the urinary bladder, not a sac. The aorta is the main artery that carries blood away from the heart to the rest of the body and is not involved in storage or muscular contraction in relation to urine. The gallbladder, while also a sac, is specifically a storage organ for bile produced by the liver, which aids in digestion rather than a part of the urinary system.