The most secure and appropriate way to share an encrypted AMI using AWS Key Management Service (AWS KMS) is to modify the launchPermission property of the AMI to share it only with the MSP Partner's AWS account. Additionally, the key policy of the customer-managed key must be updated to allow the MSP Partner's AWS account to use the key for decryption. This ensures that access is restricted to the intended AWS account only and the encryption remains intact. Other options either make the AMI publicly available, involve changing key ownership, or overly complicate the process which might introduce security risks.