As freckles are a dominant trait, if neither of the parents have freckles, which would be shown with one allele, they don't have it so they wouldn't be able to pass it to their offspring.
a. If F is the dominant allele for freckles, Olivia's genotype would be ff.
b. Marcu0s genotype would be ff too.
c. None of the chlidren have freckles.