Answer:
Step-by-step explanation:
Tsunami is usually caused by earthquake in the ocean floor. So the land-based earthquake sensor is cheap but not very useful here.
The sea floor sensor and deep water sensor will be the right choice and give the earliest warning. But they are also the most expensive.
Shallow water sensor will also detect tsunami but detection will be behind the sea floor and deep water one.
For a low cost warning system, it should consist of a large number of shallow water sensors; plus a few strategically located sea floor and deep sea sensors.