Cystic fibrosis is caused by

The correct option is A Recessive autosomal allele
Cystic fibrosis is an autosomal recessive disorder, meaning that is not inherited solely from the mother or father as in sex-linked disorders. Rather, cystic fibrosis is inherited when an individual receives a mutated copy of the gene associated with cystic fibrosis from both parents.

So, the correct answer is 'Recessive autosomal allele'.