Answer: D) Radiation.
Explanation: The shiny side of the aluminum foil is better at reflecting electromagnetic waves or radiation, so if this side faced the other side, it will prevent some radiated heat to reach the food, which is counterproductive.
When this side faces the food, it also prevents the heat radiated from the food to leave it, so it is actually beneficial.
So the correct answer of why this side should face the food is option D) Radiation