The question relates to principles of insurance, specifically focusing on the insured's responsibilities in minimizing losses. Let's break down the options to find the correct principle:

• Principle of Subrogation: This principle allows the insurer to take legal action in the insured's name against any third party responsible for the loss. This principle ensures that the insured does not profit from a loss. It is not about minimizing losses but about recovering compensation from a responsible third party.
• Principle of Utmost Good Faith: This principle mandates that both parties in an insurance contract must disclose all material facts truthfully. This principle is about honesty and transparency rather than minimizing loss.
• Principle of Co-operation: This principle relates to the mutual agreement and collaboration between the insured and the insurer, which is essential for the contract, but it primarily focuses on agreement terms, not on minimizing loss.
• Principle of Mitigation: This principle requires the insured to take reasonable steps to minimize the loss or damage to the insured property. It emphasizes the insured's duty to mitigate or reduce the potential losses even after a peril occurs.

Based on the explanation above, the principle that states it is the duty of the insured to take reasonable steps to minimize the loss or damage to the insured property is the Principle of Mitigation.

Therefore, the correct answer is Principle of Mitigation.