It expects jQuery, which, in 3.9 loads at the bottom.

You can fix this in /lib/view/WpProQuiz_View_FrontQuiz.php

By wrapping line 68 script with a load event. Hacky, but it works!

window.addEventListener('load', function() {

			jQuery(document).ready(function($) {
				$('#wpProQuiz_<?php echo $this->quiz->getId(); ?>').wpProQuizFront({
					quizId: <?php echo (int)$this->quiz->getId(); ?>,
					mode: <?php echo (int)$this->quiz->getQuizModus(); ?>,
					globalPoints: <?php echo (int)$quizData['globalPoints']; ?>,
					timelimit: <?php echo (int)$this->quiz->getTimeLimit(); ?>,
					resultsGrade: <?php echo $resultsProzent; ?>,
					bo: <?php echo $bo ?>,
					qpp: <?php echo $this->quiz->getQuestionsPerPage(); ?>,
					catPoints: <?php echo json_encode($quizData['catPoints']); ?>,
					formPos: <?php echo (int)$this->quiz->getFormShowPosition(); ?>,
					lbn: <?php echo json_encode(($this->quiz->isShowReviewQuestion() && !$this->quiz->isQuizSummaryHide()) ? __('Quiz-summary', 'wp-pro-quiz') : __('Finish quiz', 'wp-pro-quiz')); ?>,
					json: <?php echo json_encode($quizData['json']); ?>
				});
			});

		}, false);

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https://wordpress.org/plugins/wp-pro-quiz/