Tuna provides 30 calories per ounce . on a graph representing the relationship between the number of ounces of tuna a person consumes and the number of calories the tuna provides which quantity should be graphed on the x-axis

Ounces of tuna will go on the x axis and calories on the y axis.  This is because the relationship between the number of ounces a person eats and the calories he consumes then is a positive correlation (a line with a positive slope which goes from lower left to upper right) with x being the independent variable. As x increases, y increases. In other words you can ONLY change the amount of calories you consume WHEN you change the amount of ounces you eat.  You can't change the number of calories independent of the number of ounces. The independent variable goes along the x axis, cuz the x variable always represents the independent variable.