The most accurate response to the question is:
It supports the inference well because it shows something has happened that can't be reasonably explained.
The sentence suggests that the top paper was written in red ink, which is unusual since no one in the class used red ink, and it wasn't in anyone's handwriting. This creates an element of mystery and indicates the presence of unknown forces, whether supernatural or otherwise, at work. The lack of a reasonable explanation lends support to the idea of something unusual occurring.