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

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