How to add < to "allowed values list"?
Both &lt;, &amp;lt; and also &amp;#60; works with a checkbox/radio widget. &lt;select&gt; doesn't allow html code for options.

In consecuence Drupal strip tags if the &lt;select&gt; widget is configured. This is done in _options_prepare_options().

Short story: after some tests I've found that small (&lt; 100) works (there's a whitespace between &lt; and 100), so it is not considered a tag.

I've done my tests with:

drush php-eval 'print strip_tags("small (&lt; 100)") ."\n"'