Counting half-diagonals in the squares it can be seen there are 2 half-diagonals drawn in the 1st square, 4 – in the 2nd and 3 – in the 3rd one.
How many half-diagonals should be drawn in the 4th square to make the entire pattern complete?
When the square outlines are carefully highlighted or removed it can be seen three letters have been hidden in them – W, X, and Y (the row is rotated 90 degrees clockwise). Revealing this, it makes quite clear Z is missing in the last square, and thus 2 half-diagonals or a complete diagonal should be drawn in it.