Each cell within a developing body will express different genes, depending on the cell’s age and the location within the body. How is gene expression regulated?

Gene expression is the mechanism by which the information present in the DNA of a cell is transcribed and translated to form proteins.
Transfer RNA is responsible for transporting the information contained in the gene to the ribosomes that are responsible for protein synthesis.
Therefore, we can say that gene expression depends on messenger RNA. So the option "A" is correct.