Final answer:
Injecting head mRNA into the tail end of a Drosophila embryo leads to developmental abnormalities, potentially a fly with a head at both ends, due to the disruption of the orderly expression of Hox genes which control the body plan.
Step-by-step explanation:
When mRNA from the head end of a Drosophila (fruit fly) embryo is injected into the tail end of an early embryo, the result is that the fly develops abnormally. Typically, the fly might not develop properly; however, given the options, the most likely answer based on the function of Hox genes is that the fly develops a head at both ends (a), although this is an oversimplification and in reality the disruption of normal development could lead to a variety of developmental issues or lethality based on the precise nature and extent of the experimental intervention.
The body plan of Drosophila is tightly controlled by the Hox genes, and any significant changes to their expression can result in drastic effects such as the growth of appendages in the wrong places, as seen in mutants where legs grow from the head region. Moreover, changes to genes important in embryonic development are generally conserved due to magnitude of their effects in the adult organism.