When other insurance is written on the same basis as a commercial property policy, the obligation of the insurer is to cover:

When multiple insurance policies cover the same loss on the same basis, the obligation of the insurer is to cover a share of the loss. This principle is known as "pro-rata liability" or " prorata sharing." In such cases, each insurer contributes to the loss proportionally, based on their respective coverage limits.

This approach ensures that the insured does not receive more than the actual loss incurred, thereby preventing what is known as "over-insurance" or "double recovery." Instead of one insurer covering the entire loss while others contribute nothing, each insurer pays a portion of the loss based on the total amount of coverage they provided compared to the total insurance in place.

In contrast, other choices suggest that coverage could be on an excess or primary basis, which implies that one policy would pay in full while others would only pay in certain scenarios, which does not apply here given that all insurances are written on the same basis. Thus, the correct response reflects the equitable distribution of loss among all applicable policies involved.