Which of the following food items will turn blue-black in colour after we put a few drops of dilute iodine solution on them?

Carbohydrates are present in our food in the form of starch as well as sugars. We can test to check if our food contains starch in it. This can be done by putting 2-3 drops of dilute iodine solution on the food item. If it changes colour to blue-black, it means that the food contains starch in it. 

Bread and Potatoes both contain starch in them and hence will turn blue-black in colour in the presence of a dilute iodine solution. Fish and oil will not change their colour as they do not contain starch in them.