Which of the following phenotypes will react with anti-f?

The phenotype that reacts with anti-f is the rr phenotype. This is because the anti-f antibody specifically targets the "f" antigen, which is present in the Rh system. The presence of the "f" antigen is associated with the "r" haplotype, which corresponds to the rr genotype. Individuals with the rr genotype lack other Rh antigens that would lead to a negative reaction with anti-f, making them suitable candidates for showing a positive reaction.

In contrast, phenotypes like R1R1, R2R2, and R1R2 contain one or more Rh antigens that do not correlate with the "f" antigen. These genotypes produce the F antigen products, thereby leading to an absence of reactivity with anti-f. Understanding these relationships is crucial in immunohematology, as it helps in determining compatibility in blood transfusions and in understanding various blood group antigens.