Let's start by identifying the scientist's hypothesis in our own words: "high salt in the lungs of people with cystic fibrosis is the reason they cannot fight off bacteria". We want to find an answer that SUPPORTS this hypothesis.

A. When the salt concentration of the fluid on the airway surfaces of healthy people is raised artificially, the salt concentration soon returns to normal.
This tells us that healthy people are able to maintain a normal salt balance in their lungs. This tells us nothing about people with cystic fibrosis, who can't maintain a normal salt balance in their lungs. This is irrelevant to the hypothesis. Eliminate.

B. A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.
This tells us that people with low salt in their lungs could fight off bacteria. We only care about people with high salt in their lungs. This is irrelevant to the hypothesis. Eliminate.

C. When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.
This tells us that if nothing but the salt level is changed in the lungs of someone with cystic fibrosis, they can fight off bacteria. This means that nothing else in the lungs of people with cystic fibrosis is preventing them from fighting off the bacteria - the problem must be the salt. This SUPPORTS the hypothesis. Keep this answer.

D. Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.
This question discusses "many lung infections". We only care about cystic fibrosis, which may or many not be one of the lung infections addressed here. This is irrelevant to the hypothesis. Eliminate.

E. High salt concentrations have an antibiotic effect in many circumstances.
This question tells us that high salt can work as an antibiotic "in many circumstances". However, we already know that it doesn't work as an antibiotic in the cases of individuals with cystic fibrosis - they can't fight off the bacteria even with the high salt concentrations in their lungs. Remember: it isn't in question whether or not people with cystic fibrosis have a lot of salt in their lungs. This is presented as a fact in the passage. The question is whether or not this salt concentration is what prevents them from fighting off bacteria. So the "many circumstances" in this answer don't include the lungs of people with cystic fibrosis. This is irrelevant to the hypothesis. Eliminate.

The correct answer is C.