Explanation:

- The method described is when a notice about job vacancies is put up on the organization's notice board.

- Option 1: Casual callers

- This refers to people who visit the organization's premises, hoping for job opportunities, but it does not involve a notice being posted.

- Option 2: Labour contractors

- Here, contractors supply workers to the company, not through a notice board system.

- Option 3: Recommendations of employees

- This means existing employees suggest candidates, but it's not about posting job information on notice boards.

- Option 4: None of the above

- This option means that none of the previously mentioned methods fit the description given in the question. The actual method is called "Notice board recruitment" or "Direct recruitment," not any option listed.

Correct Answer: Option 4 - None of the above.