Option (i) wholesale price index is correct. The textbook context explicitly states that 'The weekly inflation rate is given by (X_t - X_{t-1})/X_{t-1} × 100 where X_t and X_{t-1} refer to the WPI for the $t^{th}$ and (t-1)^{th} weeks.' This directly shows that inflation is calculated using the Wholesale Price Index. Additionally, the context mentions that 'The Wholesale price index number indicates the change in the general price level' and that 'All Commodities Inflation Rate' (Headline Inflation) is based on wholesale prices. The CPI, on the other hand, is mentioned as being used 'in calculating the purchasing power of money and real wage', not for calculating inflation.