Recall the Stefan-Boltzmann formula:
Here / is the rate at which energy is radiated from the blackbody, is the Stefan-Boltzmann constant, is the area of the blackbody, and is the temperature of the blackbody in . So, doubling increases the power radiated by the blackbody by a factor of . Assuming the heat capacity of the water is constant over the relevant temperature range, this means that the temperature would increase instead of . Therefore, answer (C) is correct.