The answer is (b): the hole is in the center. In fact, the three black discs are, left to right, a piece of black construction paper, a hole, and a piece of black felt.
The spectrum of the radiation emerging from a hole, dependent only on the temperature of the inside the box, determines the apparent "color" of the hole. A hole is darker than an object at the same temperature, because the material of which the object is composed emits a spectrum that is dependent on the temperature but also shows some reflected light, making it brighter.
Which brings us to Question 93.
