what is an icon for the previous comment tool?

Brief Explanations:

The left - pointing arrow on an icon typically represents going back or to a previous state. In the context of a 'Previous Comment' tool, the icon with a left - pointing arrow on a document - like shape is likely to be the correct one as it implies moving to the previous comment.

Answer:

The icon with a blue left - pointing arrow on a document - like shape (the bottom - left icon in the image).