How to check in a Rules Condition if a specific value of a multi-value field is selected?
I ended up creating a "Check a Truth Value" condition with the Rules module. The code below is the one I used. 

if ($node-&gt;field_area_51[0]['value']) {
  foreach ($node-&gt;field_area_51 as $key =&gt; $value) {
    if ($value[value] == 'b') {
      return TRUE;
    }
  }
}