The correct answer is option 'C' - current period.

Explanation:
Index numbers are statistical measures used to compare the changes in the value of a variable over different time periods. They are used to track and analyze the changes in the prices, quantities, or other variables of interest.

The time period for which an index number is determined is known as the current period. It refers to the specific time frame for which the index number is calculated and represents the latest available data.

Let's understand the concept of index numbers in more detail:

Definition of Index Numbers:
Index numbers are measures that express the relative change or movement in a variable compared to a base period. They are used to simplify complex data and make it easier to understand and interpret. Index numbers provide a way to compare different time periods or different groups of data.

Components of Index Numbers:
1. Base Period: The base period is the reference period against which the changes in the variable are measured. It is assigned a value of 100 and serves as the benchmark for comparison.

2. Current Period: The current period is the time period for which the index number is calculated. It represents the latest available data and is compared to the base period to determine the relative change.

Importance of Current Period:
The current period is crucial for calculating index numbers as it provides the most recent data. By comparing the current period with the base period, we can determine the percentage change or the relative movement in the variable of interest.

For example, if we are calculating the Consumer Price Index (CPI), the current period will represent the latest month or year for which the prices of a basket of goods and services are considered. By comparing the current prices with the prices in the base period, we can determine the inflation or deflation rate.

In conclusion, the time period for which an index number is determined is known as the current period. It represents the latest available data and is compared to the base period to measure the relative change in the variable of interest.

The time period for which an index number is determined is known as the "current period" (Option C). The base period is the reference period against which the changes are measured, and the normal period is not a commonly used term in index number calculations.