This one puzzles me. I hope you can solve it.
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I think the answer is
The top left contains the letter C reflected upon itself, and the top right contains the subsequent letter, D, reflected upon itself. The bottom left contains the letter T upside-down, and tile B contains the subsequent letter, U, upside-down.
My first choice of answer wasn't here. If there had been a tile containing
the letter M reflected upon itself
I would have chosen that, because
the bottom left tile could also be read as the letter L reflected upon itself.