Final answer:
The expression of leptin in adipocytes is restricted by the promoter region of the ob gene, which ensures that gene transcription is initiated specifically in fat cells.
Step-by-step explanation:
The mechanism that restricts the expression of leptin to adipocytes is B) a promoter for the expression of the ob gene. Promoters are regions of DNA that initiate transcription of a particular gene. Adipocytes have specific promoters that are responsive to transcription factors present in those cells, which in turn initiate the transcription of the leptin gene (ob gene).
Enhancers and nuclear factors also play a role by increasing or modulating the efficiency and rate of gene expression, but the fundamental specificity comes from the promoter region which determines where and when a gene is expressed.