In Illinois, which of the following constitutes sufficient proof of loss?

In Illinois, a detailed written notice of the loss submitted to the insurer constitutes sufficient proof of loss because it provides the insurer with a comprehensive account of the incident, including the nature and extent of the damages. This written notice serves as a formal communication that not only informs the insurer about the claim but also outlines the specifics needed for the insurer to assess the situation accurately. This documentation is essential for the claims process as it helps to establish the policyholder's intent to file a claim and provides a basis for the insurer’s investigation and potential liability.

While other options may provide supportive evidence regarding the claim, such as repair cost verification or photo evidence, they do not fulfill the specific requirement for a formal proof of loss document as outlined in policy agreements. A signed statement from a public adjuster, while informative, lacks the direct context and detail that a detailed notice provides. Effective communication through a thorough written notice not only aligns with industry standards but also helps ensure the claim is processed efficiently. Thus, option C is the most appropriate choice for proof of loss in this context.