The Gross National Product (GNP) in an open economy is calculated using the following formula: * **GNP = C + I + G + (X - M)** Where: * **C** = Consumption * **I** = Investment * **G** = Government Spending * **X** = Exports * **M** = Imports * **(X - M)** = Net Exports (exports minus imports). Option D, C+I+G+(X-M), accurately reflects this formula, thus providing the correct method for measuring GNP in an open economy.