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As usual, I'm adding the link to the rules for those that don't know the puzzles Click me!

This one's easy

up vote 14 down vote accepted

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

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