If four products P, Q, R and S shown in the graph are sold at price of Rs. 9, Rs. 4, Rs.13 and Rs.3 respectively during 1990-1995, then the total revenue of all the products is lowest in which year?

Revenue of 1991 = $$\left(25\times\ 9\right)+\left(40\times\ 4\right)+\left(91\times13\right)+\left(159\times\ 3\right)=2045$$

Revenue of 1992 = $$\left(40\times\ 9\right)+\left(108\times\ 4\right)+\left(107\times13\right)+\left(165\times\ 3\right)=2678$$

Revenue of 1993 = $$\left(35\times\ 9\right)+\left(60\times\ 4\right)+\left(62\times13\right)+\left(140\times\ 3\right)=1781$$

Thus, the lowest revenue was in 1993.

Thus, the correct option is C.