marina trench is in water which is hydrosphere so how is it deepest point on lithosphere

Let's first know some facts about Mariana trench:

The Mariana Trench is the deepest point in Earth’s oceans. The bottom there is 10,924 meters (35,840 feet) below sea level. If Mount Everest, the highest moutain on Earth, were placed at this location it would be covered by over one mile of water.

Now Ans. of your actual Q:

The Mariana Trench is located at a convergent plate boundary. Here two converging lithospheric plates collide with one another. At this collision point, one of the plates descends into the mantle. At the line of contact between the two plates the downward flexure forms a trough known as an ocean trench. An example of an ocean trench is shown in the diagram below:

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