Healthy lungs produce a natural antibiotic that kills harmful bacteria.
People with cystic fibrosis, however, are unable to fight off such bacteria, even though their lungs produce normal amounts of the antibiotic.
The fluid in their lungs has an abnormally high salt concentration

Hypothesis: the high salt concentration is what makes the antibiotic ineffective.

We need to support the hypothesis. So we need to say that it is the high concentration of salt that makes antibiotic ineffective.

(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.

Doesn't tell us what happens to bacteria. Not useful.

(B) A sample of the antibiotic was capable of killing bacteria in an environment with an unusually low concentration of salt.

We don't care what happens in low concentrations env.

(C) When lung tissue from people with cystic fibrosis is maintained in a solution with a normal salt concentration, the tissue can resist bacteria.

Correct. So it seems the tissue maintains its capability to fight bacteria in all other ways. Only the high salt concentration is the problem. So when placed in normal salt concentration, the tissue fights bacteria. This strengthens that high salt is responsible for making antibiotic ineffective.

(D) Many lung infections can be treated by applying synthetic antibiotics to the airway surfaces.

Irrelevant.

(E) High salt concentrations have an antibiotic effect in many circumstances.

Irrelevant. We need to know what it is doing in this circumstance.

Answer: C