You didn't specify whether A was tackled in-bounds or OOB. If the play ended in-bounds, the clock would normally start, after enforcement, on the ready. A would have the option of having the clock start on the snap. If A declines to exercise the option leaving the clock to start on the RFP, and B's late hit penalty is accepted, NFHS 3-4-7, which allows an offended team, following an accepted penalty to elect to change the clock starting until the snap, would have that option.
If A was tackled OOB, the clock will start on the snap at the (ultimate) succeeding spot, so no clock option exists.