function validateNewPostForm()
{
var el = document.getElementById('select-subject');
alert(el.options[el.selectedIndex].label); // aaa
}
<form id="form-new-post" ... onsubmit="return validateNewPostForm();">
<select id="select-subject" ...>
<option value="a" selected>aaa</option>
<option value="b">bbb</option>
</select>
<input type="submit" id="post" value="Post" />
</form>