A hot liquid is kept in a big room. Its temperature is plotted as a function of time. Which of the following curves may represent the plot?

The temperature of the hot liquid will decrease with time. The walls of the room will absorb the emitted radiation from the liquid. Since the room is big the temperature difference between emitted radiation and absorbed radiation is large.
According to Stefan’s Law, the energy of thermal radiation emitted per unit time is directly proportional to T⁴. Thus, decrease in temperature with time will also be proportional to T⁴. Thus the curve must be exponentially decreasing curve which is curve a.
Thus, curve a represents the plot.