To solve this question, one can make use of a punnet square to determine the phenotypic ratios:
A - allele for purple colour
a - allele for white colour
Aa X Aa
Genotypes: 1 AA : 2 Aa : 1 aa
Phenotypes: 3 Purple flowered pea plants to 1 white flowered pea plant.
Assuming that this complete dominance, where the A allele, the purple colour is dominant over the lowercase a allele that codes for white colour in the pea plants.