To download an object encrypted with SSE-KMS, the IAM role must have kms:GenerateDataKey (if uploading) and kms:Decrypt (if downloading). However, for GetObject, KMS requires kms:Decrypt. But there is also a need for kms:Encrypt? Actually, reading encrypted data requires only kms:Decrypt.
However, the error might be due to the IAM role not having kms:GenerateDataKey? No, that is for writing. The bucket policy might require kms:Encrypt? Wait. The common missing permission is kms:ReEncrypt? No.
Actually, for downloading, KMS permission needed is kms:Decrypt. But the error persists. Could be kms:DescribeKey is not needed.
But the most likely is that the KMS key policy does not grant the IAM role permissions. The key policy must allow the IAM role to use the key. The question says the IAM role has kms:Decrypt, but the key policy might not grant it.
Option A is correct. Option B is for writing. Option C is not needed.
Option D is for management.