The answer is the fourth image.
If you were to fold the hexagon in half, it would be symmetrical when folded from vertex-to-vertex and when folded from the midpoint of a side to the opposite side.
This means that while the first and third images are valid lines of symmetry, they are incomplete because they're missing the second set of lines from side-to-side.
The second image also has valid lines of symmetry, but it doesn't have all of them.
Thus, the answer is the fourth image.
