Quiz

I think your suggested chagne is not quite right. I think it would be better to just delete the ' && $isselected' bit of the if test (otherwise it will give away the answer before the student has responded. So:

if ($options->correctness) {
$feedbackimg[] = $this->feedback_image($this->is_right($ans));
$class .= ' ' . $this->feedback_class($this->is_right($ans));
} else {
$feedbackimg[] = '';
}

And, yes, overriding the renderer in your theme is the right way to implement it.