questions            answers
+-----+---------+    +------+------+---------+
| id_q| question|    | id_q | id_a | answer  |
+=====+=========+    +======+======+=========+
|  1  |question1|    |   1  |   1  | answer1 |
+-----+---------+    +------+------+---------+
|  2  |question2|    |   1  |   2  | answer2 |
+-----+---------+    +------+------+---------+
                     |   1  |   3  | answer3 |
                     +------+------+---------+
                     |   1  |   4  | answer4 |
                     +------+------+---------+
                     |   2  |   5  | answer5 |
                     +------+------+---------+
                     |   2  |   6  | answer6 |
                     +------+------+---------+
                     |   2  |   7  | answer7 |
                     +------+------+---------+
                     |   2  |   8  | answer8 |
                     +------+------+---------+

this->db->SELECT('*');
this->db->FROM('answers');
this->db->WHERE('answers.id_q', $x); // $x -> variable that has the id o the selected question
this->db->UPDATE('answers', $data);

answers
+------+------+---------+
| id_q | id_a | answer  |
+======+======+=========+
|   1  |   1  | answer1 |
+------+------+---------+
|   1  |   2  | answer1 |
+------+------+---------+
|   1  |   3  | answer1 |
+------+------+---------+
|   1  |   4  | answer1 |
+------+------+---------+