The sum of food and fertilizer production has shown a constant value for how many years?

In order to find the number of years for which the sum is same, we need to determine the change in production of food and fertilizer. If there is no effective change, then the sum of their productions is same.

Change in 1984 => +1 - 1.5 = -0.5

Change in 1985 => 0

Change in 1986 => -1.5 + 1.5 = 0

Change in 1987 => 0

Change in 1988 => +1.5 - 1.5 = 0

Change in 1989 => -2.5 + 0 = -2.5

Change in 1990 => 0 + 1.5 = 1.5

Change in 1991 => 0

The change is 0 in 5 years. Hence, the answer is option D.