Official explanation
First of all, we would like to thank everyone who took the time to solve this puzzle and share their reasoning. We received many different explanations, and it was great to see how the same pattern could be interpreted in different ways.
For the official explanation published on our website, we selected three community explanations from Reddit that best matched the intended rule. Their explanations are reproduced below exactly as they were written.
Lemasney
D is the answer. In each sequence of the pattern, the umbrella has no effect and can be ignored. The two lines below start in one position, then both lines move toward the center, and finally end up in opposite positions. D completes the pattern because it follows this rule.
Black Ball
The left line moves to the right and the right line moves to the left when going from left to right. The lines rotate when going from top to bottom. The answer is D.
wrukproek
Look at every row. The lower two lines always move in the same way: the left line moves to the middle and then to the right, while the right line moves to the middle and then to the left. The top curve stays the same throughout, so the answer is D.
We sincerely appreciate everyone who participated in the discussion. Thank you for making the community such an enjoyable place to explore different ways of thinking.