Final answer:
Cystic fibrosis is a recessive genetic disorder characterized by excessively thick mucus in the lungs and digestive system.
Step-by-step explanation:
The correct option is d. Cystic fibrosis is a recessive genetic disorder. It is caused by the inheritance of two copies of the recessive allele that is responsible for the disorder.
People with cystic fibrosis have excessively thick mucus in their lungs and digestive systems, which can lead to severe respiratory and digestive problems.