Basically, ureters three coats. It has a fibrous coat, muscular coat and mucous coat.
A - Fibrous tissue
This is the fibrous coat of the ureter also known as the tunica adventitia. It is continuous at one end with the fibrous tunic of the kidney above it and at the other end, it is lost in the fibrous structure of the bladder.
B - Circular muscular fibers
The second coat is the muscular coat also known as the tunica muscularis, and based on the image, it is pointing at one of its layers, the circular longitudinal fibers. The main function of this is peristalsis, Basically it helps move the urine along the ureter.
C - Transitional epithelium
The mucous coat is made up if this. The epithelium found in this coat is called transitional because it resembles the epithelium in the bladder. It has several layers of cells and the inner most layer of cells are flattened. Now transitional epithelium stretches readily and allows distension of the organ.
D - Lumen
The lumen is lined by the mucosa of the ureter. This allows urine to pass through. The folds help it stretch so bolus of liquids can pass through. If the folds are not there, it could lead to the tearing of the ureters.
