Which of these foods would give the highest positive biuret test chicken butter coconut potatoes cookies?

The answer is chicken.

Biuret test is used to detect the presence of peptide bonds. Peptide bonds exist in proteins, so when they are present the test comes purple(positive).
Protein is found in big amouts in meat, fish and tofu.
From the options the one with the biggest amount of protein is chicken(around 33g/100g) and will cause the highest positive reaction in the biuret test.
Coconut and potato also have it, but its very little. They are rich in other macronutrients.
Cookies are full of sugar, not protein.
Butter is a source of fat, not protein.