To securely publish messages to an encrypted Amazon SNS topic, the following steps are required:

A. Resource policy for SNS topic:Ensures that the Lambda function is explicitly allowed to publish messages to the topic.

C. Resource policy for KMS key:Provides the necessary permissions to use the customer-managed key for encryption.

F. Lambda execution role:Grants the Lambda function the necessary IAM permissions to use the encryption key.

Other options:

Bis invalid because using SSE-KMS does not eliminate the need for resource policies.

Doverlaps with A, but specifying the ARN in the topic policy is covered by creating the resource policy.

Eis unrelated as API Gateway is not required for this setup.

AWS Documentation Reference:

Amazon SNS and KMS Permissions