You'll need to contact the WP Simple Survey plugin authors to fix this issue, since the problem appears in their plugin (in output_quiz.php, line 28).
Other WordPress users have reported similar issues with that plugin in the past:
http://wordpress.org/support/topic/plugin-wordpress-simple-survey-problem-with-the-code-warning-cannot-modify-header-information-headers-alrea?replies=1
http://wordpress.org/support/topic/plugin-yet-another-related-posts-plugin-conflict-with-wordpress-simple-survery-plugin?replies=2
http://wordpress.org/support/topic/error-message-141?replies=3
You can start your own thread here:
http://wordpress.org/support/plugin/wordpress-simple-survey