The soft reconfigure is correct by elimination (FGTs all support BGP Refresh, so question is not worded correctly - to refresh routes in advertisements, there is no need to do manually anything, after the change is committed to config FGT will send BGP Refresh message to the peers to notify them of it. The same is true for Cisco and Juniper routers. The question should ask "when routing policy was changed" - then yes, reconfiguraiton is the way to notify BGP peers that BGP policy was changed.
Anyway:
Graceful restart - tells BGP neighbors to keep the routes advertised by peer even when peer GOES DOWN, i.e. it contradicts with the question condition that BGP session should not go down.
Synchronization - unrelated at all, refers to the ancient rule that routes advertised/learned via BGP HAVE to be first learned via IGP (like OSPF/IS-IS/etc) to prevent loops in iBGP topology. No one uses it anymore, everyone either disables synchronization or it is disabled (as in FGTs) by default.
Dterermenistic MED - is about route preference between EBGP peers, unrelated at all to the question.