$3 per kg
1) Examining the graph, we can locate the following points
(1,3) and (3,9)
2) Since it is a direct variation, we can write that function as
y=kx
3=1*k
9=3*k
So y=3x is the rule of that function
3) Hence, point (1,3) expresses the unit rate. Then we can state
1 kg of orange costs $3
Or $3 per kg