In Drosophila the XXY condition leads to femaleness whereas in human beings the same condition leads to Klinefelter's syndrome in males. It proves:

Explanation

The question explores the different roles of the Y chromosome in sex determination across species. In humans, the presence of a Y chromosome leads to male development, which is why the XXY condition results in Klinefelter's syndrome in males. In contrast, for Drosophila, sex determination is primarily based on the number of X chromosomes, with two X chromosomes resulting in femaleness, regardless of the presence of a Y chromosome. Therefore, the correct answer is that in humans, the Y chromosome is crucial for male sex determination. Options B, C, and D are incorrect because they either misstate the role of the Y chromosome in Drosophila or misunderstand the genetic basis of Klinefelter's syndrome.