Which index is used to track the variation in the cost of essential goods and services relative to a fixed base period?

Explanation

Option A refers to the Human Development Index, which measures overall development. Option B is the Consumer Price Index, commonly used for price tracking. The correct answer is Option C, the Complete Price Index, which specifically measures price changes in basic goods and services compared to a base period. Option D, Comparative Price Index, is not the standard term used in this context.