Explanation:

- I am trying to cut down on coffee.

- This is the correct phrase. It means reducing the amount of coffee you consume.

- The phrase "cut down on" is commonly used to indicate cutting back or reducing.

- I am trying to shut down on coffee.

- This is incorrect. "Shut down" generally refers to turning off machines or stopping operations, not suitable for reducing consumption.

- I am trying to break down on coffee.

- This is incorrect. "Break down" typically means to stop functioning or to analyze something in detail.

- I am trying to turn down on coffee.

- This is incorrect. "Turn down" usually means to reject or lower the volume, not used for reducing intake.