Final answer:
The best explanation of why MyoD causes fibroblasts to express muscle-specific genes is that all cells share the same genes, and cell differentiation involves selective gene expression guided by transcription factors.
Step-by-step explanation:
The best explanation of why MyoD can cause fibroblasts to express muscle-specific genes is option (a): Unlike some other cell types, fibroblasts have not lost the muscle-specific genes from their genome. This is because all cells of a multicellular eukaryotic organism contain the same set of genes, and cell differentiation is a result of turning on some genes and turning off others.
Regulation of gene expression in eukaryotes involves multiple steps, and it is the action of transcription factors that primarily determine which genes are turned "on" or "off". MyoD is a transcription factor that can initiate the expression of muscle-specific genes in fibroblasts by binding to their regulatory regions and promoting their transcription. The fact that fibroblasts can be triggered to exhibit characteristics of muscle cells demonstrates the shared genomic content between different cell types and the pivotal role of gene regulation in cell identity.