Dear expert, how can it be lichen if it's mentioned pond water.... By the way, it's a class 12 question, not of 11.

Dear student, Lichens are indicators of pollution and they do not grow in areas which are rich in pollution that is the areas that have high Sulphur dioxide concentration. Since in this question it is mentioned that organism A survived at pH 6.4 while the same organism A didn't survive in pollution. So organism A is Lichen. We don't see whether it's pond water or not. Thanks and Regards.