If the line MN under this grid is the surface of water, which figure is the grid's water image?

Explanation

1. 01

   Water along MN, below the grid, swaps top and bottom but not left and right: a square in row r moves to row $6 - r$ and stays in its column.

2. 02

   So the shaded squares (3, 2), (4, 1), (4, 2), (4, 3), (5, 1), (5, 2), (5, 3), (5, 4) (row, column) move to (1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3), (3, 2).

3. 03

   Figure 4 shows exactly these squares.