My understanding is that storage gateway doesn’t act as a sync agent but as an hybrid architecture which may replace the SAN. But not syncing it (there is AWS DataSync for this)
So I don’t understand why answer A is the right answer
Storage Gateway has different modes and you’ll recall that File Gateway will create local network mount points for remote data…in this case, the data that you have in S3. If we bulk copy over the data, then use File Gateway as a mountpoint, it would look like a SAN to our systems. I can see where "keep the SAN in sync" might throw you off, but remember, we have to select from the available options in the exam question…not insert the ones that we would like to see. So, you’re going to look for the BEST answer among them all.
Storage Gateway can absolutely sync data…that’s how it works by syncing data from on-prem to S3…either S3 will be the primary storage (File Gateway) or local will be the primary storage (in case of Volume Gateway) with S3 as backup. Both File and Volume Gateways have a local caching concept.