If B“a”, the two Black one point skip stones are led out into the open center, and are then quite secure because they should be easily able to connect above or to the right. Almost equally important, after B“a” Black threatens to continue with B“b” to induce W“c”, then follow at either “d”, “e” or even “f” to initiate a serious attack on the baseless White group.
So the defensive W“d” would most likely follow, leaving Black with Sente to use as he sees fit because White would then have no really serious local attack available.