I'm using EE 2.9 and have the form below, can you make the Captcha field required?
{exp:channel:form channel="test" return="test/thank-you" logged_out_member_id="1" dynamic_title="[question]" status="pending" rules:captcha="required" rules:my_email="required" rules:first_name="required" rules:last_name="required"}
{label:first_name}
{field:first_name}
{label:last_name}
{field:last_name}
{label:my_email}
{field:my_email}
{label:question}
{field:question}
{label:my_nearest_branch}
{field:my_nearest_branch}
<label for="captcha">Please enter the word you see in the image below:</label>
{captcha}
<input type="text" name="captcha" id="captcha" value="{captcha_word}" maxlength="20">
<input type="submit" value="Ask Question" class="button">
{/exp:channel:form}