Based on this information, what is this plant's phenotype for the flower form trait?

 - double flowers
 - single flowers
The rose plant's genotype for the flower form gene is FF. The rose plant's genotype of FF has only F allelles. The F allele is for double flowers. So, the rose plant's phenotype for the flower form trait must be double flowers.
To check this answer, consider whether the rose plant's alleles are dominant or recessive. The allele for double flowers (F) is dominant over the allele for single flowers (f). This means F is a dominant allele, and f is a recessive allele.
The rose plant's genotype of FF has two dominant alleles. An organism with at least one dominant allele for a gene will have the dominant allele's version of the trait. So, the rose plant's phenotype for the flower form trait must be double flowers.