How to set old() to the dropdown menu ?

You will need to use the set_select() form helper. See docs

PS: If you are using any of the form_open() helpers (form_open, form_open_multipart), you do not need to use old(). Use set_value() instead.

Thanks. I'm using form helper but i tried this , but not working

PHP Code:

<select id="defaultSelect" name="nationality_id" class="form-select">
                                    <option value="">Select</option>

                                    <?php foreach ($nationalities as $nation): ?>
                                        <option value="<?= $nation['id'] ?>" <?= set_select('nationality_id') ?>><?= $nation['name'] ?></option>
                                    <?php endforeach; ?>
                                </select>