It's a little bit of anatomy, a little bit of knowing what the question wants us to know.
Here's a nice pic of the ureter under the ovary: https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary-alt:20180930192054-59433-mediumThumb-66713fig4_6.jpg?pub-status=live
Specifically, you want to protect the ureter during surgery. "Bridge over water" kind of applies here, knowing that uterine artery is also being ligated during ovary removal. It's so important for surgeons to be able to differentiate the arteries (which are ok to be sacrificed if you're doing a hysterectomy/oophorectomy) but the ureter must be kept intact.