Sickle cell anaemia is a

The correct option is B Genetic disorder

• Metabolic disorders are one cause of metabolic disorders, and occur when a defective gene causes an enzyme deficiency.
• A genetic disorder is a genetic problem caused by one or more abnormalities formed in the genome. For example, Sickle cell disease is caused by a mutation in the hemoglobin-Beta gene found on chromosome 11.
  
• Degenerative disease is the result of a continuous process based on degenerative cell changes, affecting tissues or organs, which will increasingly deteriorate over time.
• A pathogen or infectious agent is a biological agent that causes disease or illness to its host

So, the correct answer is 'Genetic disorder'.