I guess the correct answer is the color of the decorations in the room
A group of researchers wanted to determine if people will eat more food in a room with red paint and red decorations than in a room that is decorated blue. Half the participants in this study ate in a red room and half ate in a blue room. The researchers then measured how much food was consumed in each of the two rooms. In this study, the independent variable was the color of the decorations in the room.