Which of the following investigation would be diagnostic of cystic fibrosis?

A sweat test measures the amount of chloride, a pa of salt, in Sweat. It is used to diagnose cystic fibrosis (CF). People with CF have a high level of chloride in their sweat. CF is a disease that causes mucus build-up in the lungs and other organs. It damages the lungs and makes it hard to breathe Ref Davidson 23rd edition pg 944