* @param array $request $_POST data.
	 * @return WP_Error|string
	 */
	public static function quiz_answer_question( $request ) {

		$err = new WP_Error();

		$student = llms_get_student();
		if ( ! $student ) {
			$err->add( 400, __( 'You must be logged in to take quizzes.', 'lifterlms' ) );
			return $err;
		}

		$required = array( 'attempt_key', 'question_id', 'question_type' );
		foreach ( $required as $key ) {
			if ( ! isset( $request[ $key ] ) ) {
				$err->add( 400, __( 'Missing required parameters. Could not proceed.', 'lifterlms' ) );
				return $err;
			}
		}

		$attempt_key = sanitize_text_field( $request['attempt_key'] );
		$question_id = absint( $request['question_id'] );
		$answer      = array_map( 'stripslashes_deep', isset( $request['answer'] ) ? $request['answer'] : array() );

		$student_quizzes = $student->quizzes();
		$attempt         = $student_quizzes->get_attempt_by_key( $attempt_key );
		if ( ! $attempt ) {
			$err->add( 500, __( 'There was an error recording your answer. Please return to the lesson and begin again.', 'lifterlms' ) );
			return $err;
		}

		/**
		 * Check limit not reached.
		 *
		 * First check whether the quiz is open (so to leverage the `llms_quiz_is_open` filter ),
		 * if not, check also for remaining attempts.
		 *
		 * At this point the current attempt has already been counted (maybe the last allowed),
		 * so we check that the remaining attempt is just greater than -1.
		 */
		$quiz_id = $attempt->get( 'quiz_id' );
		if ( ! ( new LLMS_Quiz( $quiz_id ) )->is_open() &&
				$student_quizzes->get_attempts_remaining_for_quiz( $quiz_id, true ) < 0 ) {
			$err->add( 400, __( "You've reached the maximum number of attempts for this quiz.", 'lifterlms' ) );
			return $err;
		}

		// record the answer.
		$attempt->answer_question( $question_id, $answer );

		// get the next question.
		$question_id = $attempt->get_next_question( $question_id );

		// return html for the next question.
		if ( $question_id ) {

			$html = llms_get_template_ajax(
				'content-single-question.php',
				array(
					'attempt'  => $attempt,
					'question' => llms_get_post( $question_id ),
				)
			);

			return array(
				'html'        => $html,
				'question_id' => $question_id,
			);

		} else {

			return self::quiz_end( $request, $attempt );