Christopher is issued an insurance policy that contains an attached agreement which alters the terms of the policy. The attached agreement is called a:

The correct terminology for an attached agreement that alters the terms of an insurance policy is an endorsement. An endorsement serves to modify the original policy, adding new provisions, changing existing terms, or excluding coverage. This flexibility allows for adjustments to be made to the policy as the needs of the insured may change over time.

Understanding endorsements is crucial in the context of insurance, as they can impact coverage amounts, premiums, and specific conditions or exclusions. For instance, if an insured property has undergone renovations that increase its value, an endorsement might be added to reflect this change in value in the policy terms.

While "rider" can sometimes be related specifically to life insurance, where it adds coverage to a base policy, it is not the correct term for this context in general insurance terminology. "Supplement" and "appendix" are not standard terms used in insurance to describe modifications to a policy. Thus, "endorsement" is the most appropriate term for the scenario presented.