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?
 

When a hot liquid is kept in a big room, the liquid will begin to lose heat. Since the room is big, we can assume that the energy dissipated by the liquid will not change the temperature of the surroundings.
According to Newton's law of cooling, the rate of fall of temperature (i.e. rate of cooling) of the hot liquid is directly proportional to the temperature difference between the liquid and the surroundings of the room. Therefore with time, the temperature difference falls and so does the rate of cooling. The curve will have a decreasing slope.