Fig. shows the variation in temperature with time when some wax cools from the liquid phase to the solid phase. What does the part QS of the curve represent ?

Freezing.

In part $PQ$, the state is liquid and temperature is decreasing, In part $QS$ there is change of state from liquid to solid, i.e., this stage is freezing, without change in temperature. In part $ST$, state is solid and temperature is decreasing with time.