Purple flowers are dominant to white flowers in pea plants. A gardener mates a purple plant with a white plant and records the offspring as having all purple flowers. The gardener just performed a(n) and determined that the purple plant’s genotype was _________

Since we know the offspring's phenotype (appearance) is purple, the genotype can either be PP or Pp.
We know that the farmer crossed a white plant, we can denote it's genotype as pp and a purple plant with a dominant trait.
Because the farmer crossed the white plant with the purple plant, this tells us that the genotype of the purple plant must be PP.

Purple flowers are dominant to white flowers in pea plants. A gardener mates a purple plant with a white plant and records the offspring as having all purple flowers. The gardener just performed a(n) ________ and determined that the purple plant’s genotype was _________________