Explanation

Index numbers are constructed to show the change in a composite index value relative to a base period/year, which fundamentally relies on assuming that the components contribute additively or proportionally to the overall measure (e. g. , weighted averages). Options A and D are too restrictive; indices can use variables measured on different scales if appropriate weighting methods are used, but the core assumption is proportionality/additivity of contribution. Option C is incorrect as indices are not exclusively time-series data.