The isthmus is the narrowing portion of the uterus located just above the cervix. The term "isthmus" literally means a narrow part of a structure, which is applicable not only to the uterus but also to other anatomical features such as the uterine tubes, colon, and ureters. Understanding this terminology is crucial for identifying various parts of the reproductive system.

In contrast, the lumen refers to the cavity within an organ, such as the cavity of the uterus where a fetus may develop. The cervical canal is the passage that constitutes the cervix itself, and it is not situated above the cervix. Additionally, the fundus is the rounded top portion of the uterus. Therefore, when asked about the narrowing portion of the uterus, the correct answer is B, the isthmus.