The phenotype of the parents will be all red.
The monohybrid snapdragons are generating red and white seeds in the ratio 3: 1 with the white (r) phenotype being recessive and the red (R) phenotype being dominant. Thus, they would possibly have the parents exhibiting genotypes, Rr and Rr, that is, both red.
The gametes generated by these parents are R and r. The cross of these parents will lead to the formation of the following generation shown in the cross below, that is, in the ratio 3: 1 (red: white).