The sentence in question:
Every list item that is marked with an
The word “asterisks” isn’t spelled out, so I’m not sure if “an” or “a” is the correct word to put before it.
The usage should depend on how the symbol would be spelled out.
* is an asterisk. It is proper to say this is “an asterisk” so you would say an *.
This is due to the way you would read it out loud.
When reading: Every list item that is marked with an * is optional.
I would say out loud: Every list item that is marked with an asterisk is optional.
Therefore an must be used.