if (have_rows('your-repeater')) {
  ?>
    <select>
      <?php 
        while (have_rows('your-repeater')) {
          the_row();
          ?><option value="<?php the_sub_field('your-sub-field'); ?>"><?php the_sub_field('your-sub-field'); ?></option><?php 
        }
      ?>
    </select>
  <?php 
}