Step 1: Understand the term.
The hydrological cycle is just the water cycle. It is the way water keeps moving between the land, the air, and the sea through evaporation, clouds, rain, and flow back to rivers.
Step 2: What controls this cycle.
We are asked which living thing helps run this cycle the most among the choices given.
Step 3: Look at forests.
Trees pull up water from the soil and release it into the air as vapour. This release is called transpiration. Together with plain evaporation it is called evapotranspiration.
Step 4: Other roles of forests.
The leafy tops of trees catch the rain and slow it down. This lets more water soak into the ground instead of running off fast. So forests balance the flow of water.
Step 5: Compare the other options.
Lichens, epiphytes, and grasslands do add a little moisture, but they cannot match the large scale effect of big forests.
Step 6: Conclusion.
Forests act like a giant pump and sponge for water, so they control the hydrological cycle.
Answer: Forests