In order to solve this we can start by formulating the average for the given set of values. The average is calculated by adding the numbers of the set and dividng the result by the number of values. In this case we have the following set of values: j, j, k and n, since there are four of them we get the following expression for the average value:

We are told that the average value equals 0, then by replacing 0 for av, we get:

By multiplying this expression by 4 we got 2j + k + n = 0, if we subtract 2j on both sides of this equation we get:
2j + k + n = 0
2j - 2j + k + n = -2j
0 + k + n = -2j
k + n = -2j
Then, option C is the correct answer, k + n = -2j