Calculating future data based previous data 
The sales volume of the next month is predicted by the data in the
  past.   The sales  volume is changed greatly from month to month, but
  the annual fluctuation pattern is  almost the same every year.   Which
  of the following is the most appropriate formula  that can be used for
  calculating the sales volume of the next month?   Here, Pt+1 is  the
  sales volume predicted for the next month, St is the sales volume of
  the current  month t, and the data is retained for three years.

a) Pt+1 = (St+ St–1+ St–2) / 3  
b) Pt+1= St xSt / St–1
c) Pt+1= (St+ St–12 + St–24) / 3  
d) Pt+1= (St–11+ St–23+ St–35) / 3  

I would love to know the right method to solve this one, some explanations will be greatly appreciated! 

Answer d may be the best option, since it calculates the mean of the values observed in the successive month during the previous 3 years. The information that sales volume changes greatly from month to month, but with the same annual fluctuation pattern every year, suggests that a reliable prediction can be based on the same month observed in the previous years.
