A population is composed of 100% Rr individuals,what is the frequency of the r allele?

If all the individuals in the population are Rr, then the frequency of the r allele is 50% (because it shows up as one of the two alleles in every individual). Given time and random breeding, and uming the frequency of r didn't change, the population would eventually become 25% RR, 50% Rr, and 25% rr.