Mendelian disorders are caused by

The correct option is B mutation in the single gene
Mutation is a phenomenon that results in changes in the nucleotide sequences of the genes in the genome. Mendelian disorders are caused by mutations in the single gene. These disorders are transmitted to the offspring from parents as per the Mendelian laws of inheritance.

The Mendelian laws of inheritance state that genetic characters are controlled by unit factors (genes) which exist in pairs in organisms. When two unlike factors responsible for a character are present, only one of the factors (dominant) expresses itself while the other remains suppressed (recessive). During gamete formation, the paired unit factors segregate randomly such that each gamete receives only one factor. Each factor segregates independently such that every gamete receives one or the other factor with equal possibilities.

Some examples of Mendelian disorders are haemophilia, cystic fibrosis, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia, etc.