Answer:
Mutation affecting gene expression is the most likely reason for a fruit fly to develop legs on its head instead of antenna.
Explanation:
WHAT IS MUTATION:
Mutation is the process whereby an alteration or change occurs to the sequence of DNA either by mistake during replication or caused by mutagens.
WHAT IS GENE EXPRESSION?
Gene expression is the process by which the information encoded by genes are used to synthesize products such as proteins etc.
In the organism described in this question, the fruit fly develops legs on its head instead of antenna, it's most likely because the genes encoding the information for the synthesis of antennas on its head have been CHANGED OR MUTATED, hence, causing something else to be formed there.
Therefore, mutations affecting gene expression can cause legs to be formed on the head of fruit fly instead of antenna.
