morepurplemorebetter
Explorer
There is no option, because the intention behind it is that you simply write "10" in the AC/Armor Bonus field, and leave the description field blank. At least, that is how I prefer it and how it was originally intended. In the original, leaving the description field blank would have the AC/Armor Bonus field auto-populate with a "10". However, somebody along the line (@JValeur to be exact) correctly pointed out that some people would like a way for the armor AC bonus and the total AC field to remain empty while printing. Thus I made both fields empty by default.I've always found it odd that there is no option in the Armor dropdown to choose "no armor" or something similar. As a Sorcerer I am not proficient in any armor and at low level I don't even have Mage Armor unless I had picked it as one of the starting spells (which I didn't). Or just that having the armor description blank it automatically assumes you have 10+Dex in AC instead of waiting for a second number other than Dex to calculate your Armor Class.
So what you are asking is already possible, just not very well explained or part of the drop-down box. If you write either "unarmored", "nothing", "naked", or "bare" in the armor description drop-down box, you will see that the sheet auto-populates a 10 in the armor AC bonus field. However, I fully understand your request and will be adding "unarmored" to the drop-down box. I will also add "no armor" as a recognized option. I'll make these changes in the next version!
Please note that none of the drop-down boxes on the sheet are limited by their options, you can always type in anything you want into the field, the list is there just to make the selection quicker. Also, you can manually set the armor AC bonus to any number you want.