Cystic fibrosis can be treated by .......... gene rapy.

Cystic Fibrosis (CF) is an autosomal recessive genetic disease that affects the lungs. It is characterized by the thick and sticky mucus due to defective gene. It occurs due to mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. It can be treated by gene therapy using DNase. DNase is a DNA-destroying enzyme that breaks DNA strands remained by dying cells. DNase I reduces sputum viscosity in the lungs of CF patients as viscous sputum causes recurrent infections and affects lung function. 

So, the correct option is 'C'.