Answer:
Cross an individual of the F1 with the purebred red flower, round pollen sweet pea parental.
Explanation:
The parental individuals are:
The resulting F1 of the cross between the different parental types can only be PL/pl, which has purple flowers and long pollen. This means that the purple flower (P) and long pollen (L) traits are dominant.
To determine the recombination frequency between genes P/p and L/l, the cross should be done between a heterozygous individual for both genes (the F1) and a homozygous recessive individual for both traits (the pl/pl parental). This is called a test cross.