Final answer:
The female sex-determining pathway in Drosophila melanogaster is initiated by the SXL gene through alternative splicing, making the correct answer D) SXL; alternative splicing.
Step-by-step explanation:
Having two copies of the X-chromosome gene, SXL (Sex lethal), initiates the female sex-determining pathway through a specific gene regulatory mechanism. In Drosophila melanogaster, a series of splicing events determine sex on a cell-by-cell basis, and Sxl is a crucial gene for initiating female development. The SXL protein is a splicing repressor which binds to the pre-mRNA of the transformer (tra) gene. Alternative splicing of tra in the presence of SXL results in female-specific transcripts that lead to female sexual development, whereas in males this alternative splicing does not take place. Therefore, the correct answer to this question is D) SXL; alternative splicing.