The answer is heterozygotes Rr with red flowers.
Dominant red flower (genotype RR) can give only dominant R allele to the offspring.
Recessive white flower (genotype rr) can give only recessive r allele to the offspring.
The cross will look like:
Parents: RR x rr
Offspring: Rr Rr Rr Rr
Offspring with Rr is heterozygous. In dominant traits, at least one dominant allele is necessary for the expression of the trait. So, heterozygotes express dominant trait: offspring with Rr genotype has red flowers.
