Use the method correct_response from render.php.

question.php

@@ -365,34 +365,35 @@ class qtype_stack_question extends question_graded_automatically_with_countback
     }
 
     public function format_generalfeedback($qa) {
+        if (empty($this->generalfeedback)) {
+            return '';
+        }
+
+        $gftext = new stack_cas_text($this->generalfeedback, $this->session, $this->seed, 't', false, true);
+
+        if ($gftext->get_errors()) {
+            throw new stack_exception('Error rendering the general feedback text: ' . $gftext->get_errors());
+        }
+
+        return $this->format_text(stack_maths::process_display_castext($gftext->get_display_castext()),
+                $this->generalfeedbackformat, $qa, 'question', 'generalfeedback', $this->id);
+    }
 
-        $feedback = '<hr>';
+    public function format_correct_response($qa) {
 
     // We need to make sure the inputs are displayed in the order in which they
     // occur in the question text. This is not necessarily the order in which they
     // are listed in the array $this->inputs
+    $feedback = '';
     $inputs = stack_utils::extract_placeholders($this->questiontextinstantiated, 'input');
     foreach ($inputs as $name) {
         $input = $this->inputs[$name];
         $feedback .= html_writer::tag('p', $input->get_teacher_answer_display($this->session->get_value_key($name), $this->session->get_display_key($name)));
     }
 
-        if (empty($this->generalfeedback)) {
     return $feedback;
     }
 
-        $feedback .= '<hr/>';
-
-        $gftext = new stack_cas_text($this->generalfeedback, $this->session, $this->seed, 't', false, true);
-
-        if ($gftext->get_errors()) {
-            throw new stack_exception('Error rendering the general feedback text: ' . $gftext->get_errors());
-        }
-
-        return $this->format_text(stack_maths::process_display_castext($feedback.$gftext->get_display_castext()),
-                $this->generalfeedbackformat, $qa, 'question', 'generalfeedback', $this->id);
-    }
-
     public function get_expected_data() {
         $expected = array();
         foreach ($this->inputs as $input) {

renderer.php

@@ -430,4 +430,9 @@ class qtype_stack_renderer extends qtype_renderer {
         $result = new stack_potentialresponse_tree_state(1, true, $fraction);
         return $this->standard_prt_feedback($qa, $qa->get_question(), $result);
     }
+
+    public function correct_response(question_attempt $qa) {
+        $question = $qa->get_question();
+        return '<hr />'.$question->format_correct_response($qa);
+    }
 }