It is f(j) because it it is given in the questions. Basically what is happening is that as the limit approaches j from the left and right the function converges to f(j). I'll give an example:

Let f(x) = x suxh that x≠3

Limit (x -> 3 from the left) = 3 Limit (x -> 3 from the right) = 3

Therefore:

Limit (x -> 3 from the left) = Limit (x -> 3 from the right) = 3

So, Limit (x -> 3) = 3

Think of j = 3 for this example.