Atemperature of 273 k is the temperature at which water
a. freezes.
b. evaporates.
c. boils.
d. lacks any moving molecules.

The answer is a, freezes. 273 K = 0 degree Celsius. It is the melting point of ice, and freezing point of water. 

BUT at this temperature, it can also remain as just 0 degree Celsius water, so it actually depends on the context of your question. 

The more accurate answer is that it is the temperature at which water freezes under normal altitude and conditions.