Final answer:
Experiments with cloning have shown that all cells in an organism, whether specialized or not, contain the same DNA, but differ in gene expression due to gene regulation. The option (A) is correct.
Step-by-step explanation:
Experiments involving cloning have demonstrated that differentiated cells contain all of the genetic information found in undifferentiated cells. This was evidenced by the famous experiment conducted by John Gurdon and Shinya Yamanaka, who cloned frogs by transferring nuclei from differentiated intestinal cells into enucleated eggs.
The resultant embryos developed into normal tadpoles, showing that no genes were lost during the differentiation of cells. Thus, all cells in a multicellular organism, despite their differences in size, shape, and function, contain the same DNA but differ in which genes are expressed. Gene regulation, therefore, is the key process that leads to the unique genetic expression of each cell type and allows for the differentiation of cells within an organism. Therefore, option (A) is correct.