Step 1: Start from the formal definition of hydrology.
Hydrology is the earth science dealing with the water of the planet, its existence, movement and distribution, both on the surface and underground, and its interaction with living things and the environment.
Step 2: Test whether each listed aspect is essential to that definition.
Occurrence covers where water is found, oceans, atmosphere, ice, surface streams and groundwater. Distribution covers how much of the total water resource is present in each of these reservoirs at a given time. Circulation covers the continuous movement of water between these reservoirs through evaporation, condensation, precipitation, runoff and infiltration, which is the hydrologic cycle itself.
Step 3: Check that none of the three can be dropped.
A treatment of hydrology that ignored circulation could not explain runoff, infiltration or evapotranspiration, all of which are core hydrology topics, so leaving out any one of occurrence, distribution or circulation would make the definition incomplete.
Step 4: Conclude.
Since occurrence, distribution and circulation are all genuinely part of what hydrology studies, the correct answer is (A), (B) and (C).
\[ \boxed{(A), (B) and (C).} \]