With the following crosses of pea plants, give the flower color of offspring and the ratio expected (red is dominant). A cross of red x red (both homozygous) would result in

So, both pea plants being crossed are homozygous reds, which means that their alleles look like: RR. This means that there are only R alleles present, so you can't have any other kind of flower without a genetic mutuation.
This means that the offspring must have a red flower, which would occur 100% of the time.