Analyze the inheritance of flower color in a plant, including identifying the recessive trait, explaining its inheritance pattern, predicting offspring genotypes from a self-cross of recessive individ

The recessive trait is white flowers.

When two red-flowered plants are crossed and produce both red and white offspring, the white trait must have been masked in the parents. Recessive traits are only expressed when two copies of the recessive allele are present, and they reappear in the offspring of heterozygous parents.

Let R represent the allele for red flowers (dominant) and r represent the allele for white flowers (recessive).
White flowered plants have the genotype rr.
If a white flowered plant (rr) is selfed, the cross is rr $\times$ rr.

A test cross is a genetic cross between an individual with a dominant phenotype but unknown genotype and an individual that is homozygous recessive for the trait in question. It is used to determine if the unknown individual is homozygous dominant or heterozygous.