If you cross homozygous dominant red flower (RR) with a homozygous recessive white flower (rr), what are the possible genotypes and phenotypes of the offspring?

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.