The GDP deflator is a measure of the level of prices of all new, domestically produced, final goods and services in an economy. It is calculated as the ratio of Nominal GDP (GDP at current market prices) to Real GDP (GDP at constant base-year prices), multiplied by 100. Nominal GDP reflects the market value of goods and services using the prices of the current year, while Real GDP adjusts for price changes using prices from a chosen base year. The deflator essentially isolates the price effect from the volume effect in nominal GDP. Option A, B, and C describe incorrect ratios or use incorrect terminology. Specifically, option B is the inverse of the correct ratio definition.