From the given graph let's select all input values for which f(x) = 6.
Here, we are to select all values of x when y = 6.
From the graph, when y = 6, there is no input value of x. This is because the line does not extend to the point y = 6.
Therefore, the input values for whcih f(x) = 6 does not exist in the graph.
ANSWER:
d. None of the above