Step-by-step explanation:
The correct answer is d, Different cell types have different regulatory transcription factors. In eukaryotic cells, the expression of certain genes are controlled by regulatory transcription factors, which can determine which set of genes will be expressed in certain types of cells. These transcription factors bind to specific DNA sequences known as promoters and enhancers, which are located near the gene that they regulate. By binding to these regions, transcription factors can control the levels at which a gene is expressed, thus allowing for different cell types to yield different gene expression profiles.