In humans, having brown hair is dominant to having blond hair. How is it possible that two brown-haired parents have a blond-haired child?

a) Both parents have brown hair but their genotype is homozygous.
b) Both parents have brown hair but their genotype is heterozygous.
c) One parent has an allele for blond hair that was passed on to the child.
d) The brown hair alleles changed to blond hair when inherited by the child.