Explanation:

- Trade union security measures are policies that ensure union stability and influence within workplaces.

- Option 1: Closed shop system - Requires employees to be union members before hiring, thus securing union presence.

- Option 2: Agency shop system - Permits non-membership but mandates contribution to the union, offering financial security.

- Option 3: Open shop system - Correct. This system allows employees to freely choose whether to join or support the union financially, providing no specific security to the union.

- Option 4: Union shop system - Requires employees to join the union post-employment, enforcing union membership.