In meteorology (the study of weather), a "front" is the leading edge
of an air mass, that's moving in and replacing a mass of a different
kind of air. Like warm moving in and replacing cold, or cold moving
in and replacing warm.
Remember that cold air is heavier than warm air. So whenever a
warm or cold front moves in, the cold air always stays low and the
warm air always lifts up over it.
With that little bit of an explanation, you can see that choice-D is
the only choice that makes sense.
