NFHS Question need ruling

[zebra5150]

Ok guys here is the situation.......

3rd and 6 on team A’s 20 yd. line. The ball is snapped and A1 throws a legal forward pass caught by A2, who advances to the B 36 yd. line. After the play, the officials realize that B had 12 men on the field. The 12th man is B12 who (a) was on the field at the snap, but realized it shortly after and left the field at his team box, never participating in the play, or (b) remained on the field during the play, although he just stood there, or (c) entered the field after the snap because he thought his team only had 10 players on the field. What is the foul and enforcement?

NFHS ruling guys... thanks