Answer:
D) loss of heterozygosity
Step-by-step explanation:
In sporadic cases of retinoblastoma, only two gene mutations are thought to be necessary in the same cell for a tumor to develop. This happens when an organism inherits a mutant allele and a wild-type allele forms the parents. When the wild-type allele is mutated, resulting in tumorigenesis, this process is called loss of heterozygosity.