Answer:
Since both plants are true breeding, it means that both are homozygous for the allele of the trait, that is flower color. The geranium plant with red flowers can be denoted as RR and the plant with white flowers can be denoted by WW. Since it is not mentioned in the question that which trait is dominant over the other, but one thing is sure that all the offspring will look alike in color, because their genotype will be same, RW.
The punnet square that best describes them is in attached figure, please see.
Hope it helps!