The sum insured under the unspecified items section of a personal possessions policy extension is:

In the context of a personal possessions policy extension, the sum insured under the unspecified items section, often referred to as a 'bedroom-rated' policy, is typically selected by the insured. This means that the policyholder decides how much coverage they believe is adequate for their unspecified possessions, which may include items like jewelry, cameras, and other valuables that are not listed specifically in the policy.

This flexibility allows policyholders to tailor their coverage to better reflect their personal belongings and their value, ensuring they have adequate protection without needing to provide detailed lists of each item. Choices like a fixed percentage of specified items or contents sum insured impose limits on coverage that might not align with the individual's unique circumstances and values. Thus, the correct option accurately represents the arrangement and choice that personal possession policyholders have regarding their unspecified items.