Hi
Sorry to hear about your exam. Just keep trying! Do you have the PPL confuser? Its worth getting if not. I also used AirQuiz online which was also a very good source for revision. I had the same question in my met exam. Remember that visibility does not improve as such, it only changes 'visibility range'. Flying with the sun behind improves visibility range, and flying towards the moon improves visibility range. The reason vis range improves when flying towards the moon is due to the silhouette of objects. I think the question you answered although probably phrased slightly differently is correct.
I would re-read your met book again and try some more practice questions either using PPL confuser or AirQuiz. I managed to get an average of 96% in my exams using a combination of the above and I also wrote all of the questions from PPL confuser in a note book and made side notes. If I wasn't sure about the answer I highlighted it and re-revised. My own technique which I have always used for exams and it hasn't let me down yet!
Good luck