Recall that when the observed lapse rate is small (i.e. there is little temperature change with height in the atmosphere) the stability is high. So, in order to make the atmosphere more stable, we need to figure out which situation makes the temperature difference between two different levels of the atmosphere SMALLER. Heating the ground during the day actually increases the temperature difference between the surface and say 1 KM above the surface. When cold air is moved over a warm lake, the same thing happens. Finally, if you cool the middle troposphere (500 mb) but keep the surface temperature the same, the difference between the two levels increases. Ony when you cool the surface will you DECREASE the temperature difference between the surface and 1 KM. So the answer is A.