The narrowest pa of the ureter is at:

The narrowest lumen is where the ureter enters the bladder wall (ureterovesicle junction) and may be responsible for arresting the passage of stones of as little as 2-3 mm. Note: Sometime the question may not have the option of ureterovesical junction, in that case the intramural ureter (the pa inside detrusor) may be taken as the answer.