Final answer:
From this experiment, where true-breeding fruit flies with brown and red eyes produced all F1 offspring with red eyes, we can conclude that the red eye color allele is dominant to the brown eye allele.
Step-by-step explanation:
The experiment in question involved the crossing of fruit flies from true-breeding lines with brown eyes and red eyes. The F1 generation had red eyes, which allows us to conclude that the allele for red eyes is dominant over the allele for brown eyes. This is because in the presence of a dominant allele, the trait expressed by the dominant allele will be manifested in the heterozygote offspring, which is exactly what was observed in the F1 flies.
It is important to note, however, that this determination is based solely on the information given in the question. In general, eye color in Drosophila melanogaster can be an X-linked trait. But because the question specifically states that the locus is autosomal, and since we do not have any information indicating that these alleles underwent independent assortment or that the gene is X-linked, the correct conclusion is that the allele for red eyes is dominant to the allele for brown eyes.