The answer is A -The red geranium is heterozygous for red flowers (Rr).
We know that the red flower, to be red needs to have at least a dominant allele, so it could be either Rr or RR.
Let's try both scenarios.
If the white geranium (rr) is being crossed with RR, all the offspring would be red with genotype: Rr.
On the other hand, if the flower is Rr and it's being crossed with rr, that would result in half of the new generation plants having red flowers and the other half white flowers.