How can I retrieve data within my widget

Read:
SELECT_a_Column

$sql = "SELECT 'liters' FROM " . $wpdb->prefix . "tfp";

$liters = $wpdb->get_col( $sql );
if( $liters ) { 
  foreach( $liters as $liter ) { 
        echo '<option value="' . $liter . '">' . $liter . '</option>';
  }
}

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