From which of the following observations did Morgan conclude that the gene for eye colour in Drosophila was located on the X chromosome?

The correct option is C Presence of only males with white eyes in the $F_2$ generation

In the monohybrid cross that Morgan conducted with Drosophila, he chose to study the eye colour. In the parental generation, he crossed red eyed female with white eyed male. At that time, it was not known whether the gene for eye colour was located on autosomes or sex chromosomes. When he obtained the $F_1$ generation, he noticed that all the offspring had red eyes. Thus, he concluded that red eye colour is dominant over white.

He further crossed the $F_1$ males and females. In the $F_2$ generation, he obtained red and white eyed flies in the ratio of 3:1. He repeated the crosses several times and noticed that the white eye trait appeared only in males in the $F_2$ generation. No females with white eyes were born. He concluded that the male and female flies inherited the traits in different patterns, which matched the inheritance pattern of X chromosome.