Answer: I think that the answer is cookies.
Explanation:
The least common denominator for the fractions would be 840.
Bread: 2/8 x 105/105 = 210/840
Pie: 1/7 x 120/120 = 120/840
Cookies: 1/3 x 280/280 = 280/840
Cake: 1/5 x 168/168 = 168/840
Least to greatest:
120/840, 168/840, 210/840, 280/840
That is how I know that the cookies use the most oil.
I got this information from Calculator Soup so this isn’t all of my information.