After activating Basri's second ability, a creature that attacks gets a +1/+1 counter before blockers are chosen.
2020-06-23
Basri's last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't get +2/+2 or gain flying.
+1: Put a +1/+1 counter on up to one target creature. It gains vigilance until end of turn.
−1: Whenever a creature attacks this turn, put a +1/+1 counter on it.
−6: Creatures you control get +2/+2 and gain flying until end of turn.
