Проверьте **** Измененную линию **** ниже. Используйте fetch_assoc вместо mysqli_fetch_assoc
/*
* Get total questions
*/
$query = "SELECT * FROM questions";
//Get result
$results = $mysqli->query($query);
$total = $results->num_rows;
/*
* Get correct option
*/
$query = 'SELECT * FROM options WHERE WHERE question_no = $number AND is_correct = 1';
// Get result
$result = $mysqli->query($query);
// Get row
$row = $result->mysqli_fetch_assoc(); // ****Changed Line****
//Set correct choice
$correct_choice = $row['id'];
//Compare
if ($correct_choice == $selected_choice) {
//Answer is correct
$_SESSION['score']++;