Explanation:

- Establishing physical barriers over investment securities is crucial. It prevents unauthorized access and theft.

- Maintaining files of authorized signatures helps verify who is permitted to handle or authorize transactions, reducing the risk of fraud.

- Segregating investment approval from accounting and custody avoids conflicts of interest and ensures checks and balances. This separation is essential for internal control systems.

Correct Answer: Option 4: All of the above

These combined actions strengthen overall security by addressing physical, procedural, and personnel risks.