Which square doesn't belong here and why?
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Only number 1...
has lines that meet the bounding square at a corner
Only number 2...
lacks any lines that slant down and to the right
Only number 3...
(a) divides the square into 6 areas instead of 5, (b) contains two complete intersections, and (c) includes a complete red triangle
Only number 4...
has no uniquely distinguishing characteristics. So my guess is 4.
frame 3 because is the only one that shows the triangle created by the three lines through their pairwise intersections. In all four cases the (non-trivial) triangle exists, but in cases 1, 2 and 4 it is not presented within respective frame. (Edit: note that this is technically equivalent to AeJey's answer that observes the number of parts the frame is split into by the three lines.)
The answer is;
For three reasons:
When you extend the lines of 1, 2, and 4, they can join in a path of 3 lines which are in broken. You cannot do this with 3.
When you look at how many sections of white space there are in 1, 2, and 4 there's 5. In 3 there's 6
3 is the only square, where the lines within the frame make a triangle.
So obviously it has to be the answer stated above!