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: \[ \boxed{to \; measure \; the \; general \; changes \; in \; the \; prices \; of \; goods.} \]
Calculate price index number from the following data:
\[ \begin{array}{|c|c|c|c|c|} \hline \text{Commodity} & A & B & C & D \\ \hline \text{Price in 2005 (₹)} & 6 & 16 & 24 & 4 \\ \hline \text{Price in 2010 (₹)} & 8 & 18 & 28 & 6 \\ \hline \end{array} \]