Concept:Index numbers measure changes in economic variables; weighted formulas assign importance to items and are most frequently used.Explanation:Weighted index numbers (e.g., Laspeyres, Paasche, Fisher) account for the relative importance of different items.They are more accurate and commonly applied in economic and business contexts.Unweighted formulas are simpler but less precise, and fixed weight formulas are a subset of weighted ones.Thus, the most frequently used formulae are weighted.Answer:A. weighted formulae