As usual, I'm adding the link to the rules for those that don't know the puzzles Click me!
This one's easy
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It seems to me that
The diagrams on the left have exactly one point of intersection of degree $3$ while those on the right have either a point of intersection of degree $4$ or two points of intersection of degree $3$. Here, the degree of a point of intersection is the number of curves emerging from it in any given direction.
See the following diagram, for illustration