In fruit flies, the gene for red eyes (r) is dominant and the gene for sepia eyes (r) is recessive. if a red-eyed heterozygous fly (rr) mates with a sepia-eyed homozygous fly (rr), what are the possible genotypes of their offspring? (hint: you many want to complete a punnett square.)

rr only

rr, rr, and rr

rr and rr

rr and rr

The answer is Rr and rr.

We know:
r - recessive allele
R - dominant allele
RR - dominant homozygote with red eyes
Rr - heterozygote with red eyes (since one dominant allele R can mask recessive allele r)
rr - recessive homozygote with sepia eyes

Heterozygote Rr can give to the offspring either dominant allele R or recessive allele r. Recessive homozygote rr can give to the offspring only recessive allele r (take a look at the uploaded image of Punnett square).
After crossing the parents:
Parents:        Rr    x     rr
Offspring:  Rr    rr    Rr    rr

So:
2 out of 4 offspring will be with heterozygous with red eyes Rr: 2/4 = 0.5 = 50%
2 out of 4 offspring will be with homozygous with sepia eyes rr: 2/4 = 0.5 = 50%