Impressions of leaves are recognized evidence of what pre-existing

The correct answer is - plants.

The impressions of leaves are a recognized evidence about the existence of plants plants in the geological past of the Earth, at least from certain periods onward.

The leaves themselves are very easily disposable, so it is pretty much impossible for them to be preserved in their natural form, or at least close to it. On the other hand, in certain conditions, the leaves that were disposing, managed to leave impressions, and they were kept in certain types of rocks, mainly sedimentary ones, thus providing us with evidence of their existence in the past.