What should be entered in Block 9 C if Block 9 D is filled out?

When Block 9 D of the CMS-1500 form is completed, it indicates that there is a secondary insurance involved, which is typically a Medigap policy. In this instance, Block 9 C should be left blank. This is because Block 9 C is specifically designated for the "Other Insured's Policy or Group Number," and when Block 9 D is utilized to document a different insurance plan, there is no need to fill Block 9 C. The form is structured to provide clear information about the primary and secondary insurers, and leaving Block 9 C blank maintains this clarity.

Entering the Medigap insurer's address, writing 'Not Applicable', or including the primary insurance address would introduce confusion and potentially delay the processing of the claim by insurance providers. Therefore, keeping it blank is the correct and appropriate action in this scenario, as it accurately reflects the required information on the claim form.