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Answer:
Enhancer sequence.
Explanation:
Enhancers may be defined as the transcription factors that are used to enhance or increase the activity of the gene. Enhancers bind with the transcription factors to regulate and increase its expression.
The mutation in the enhancers decrease the transcription of  of the other gene. These sequences lies far away from the transcription site but still known to increase the rate of transcription. Hence, the mutation has been done in the enhancer sequences.
Thus, the correct answer is option (b).
 
        
             
        
        
        
A. Duplication is one of the different types of chromosomal mutations.
        
             
        
        
        
Lipids are found in cell membranes in double layer which is called "Lipid-bilayer".
So, option A is your answer.
Hope this helps!