Which is NOT one of the duties following a loss that a policyholder must fulfill?

The duty not to protect the property from theft after a loss is correct regarding the responsibilities of a policyholder. Typically, a policyholder is expected to take reasonable steps to safeguard their property from further damage or loss, which includes ensuring the property is secure from theft. Meanwhile, the other duties listed—providing prompt notice of the claim, submitting a proof of loss, and assisting the insurer during the investigation—are fundamental to the claims process. These actions help facilitate the insurer's ability to assess the claim accurately and efficiently. Therefore, while it is important for a policyholder to be proactive in protecting their property, the specific duty of protecting against theft is not universally mandated as a responsibility in many insurance policies following a loss, making it the correct choice in this question.