A classmate is working with a source that is labelled 18 V. The classmate refers to
the source as a cell. Why might you think
that the term cell is incorrect? How is the
source most likely related to a cell?​

The term cell is incorrect because, the average voltage of a cell is 3.40 V and as already mentioned in question, the volt of source is 18 V, so cell couldn't be the appropriate word, actually the source is battery.

Now, the source is most likely related to cell because a battery which is the source here, is made up of a "group of cell".

i guess that visual aids present persuasive examples to what you are presenting, some factors would be theme (does it fit with the color, information, what you are presenting), size (can you see it, is it a big enough span of data/information), and overall look of the visual aid.

hope this : )

their crystal patterns have been disrupted by stress