Step 1: Define the Price Index.
The Price Index is a statistical measure that shows how the average price of a basket of goods and services changes over time, reflecting the general changes in the price levels.
Step 2: Analyze the options.
- Option (a) is correct because the Price Index measures changes in the general price level of goods.
- Option (b) is incorrect because the Price Index is not used for geographical measurements.
- Option (c) is incorrect as the Price Index is not related to air pressure.
- Option (d) is incorrect as it is not used for measuring individual goods.
Step 3: Conclude.
Therefore, the correct answer is that the Price Index number is used to measure the general changes in the prices of goods.
Final Answer: \[ \boxed{to \; measure \; the \; general \; changes \; in \; the \; prices \; of \; goods.} \]