What is the probability that a seed from a parent plant of genotype Rr will carry the r gene

The answer is 50 percent.

Parent plant of genotype Rr will in meiosis produce haploid gametes which will carry either R allele or r allele. Since there are only two possibilities R or r, the probability that seed will get r gene is 1 of 2. This can be presented as 1/2, which is 50%.