Professor's Warning is a common Instant card from Strixhaven: School of Mages (STX), collector number 84. Current market price: $0.30 (foil $0.28), TCGplayer market prices via Scryfall.
Choose one — • Put a +1/+1 counter on target creature. • Target creature gains indestructible until end of turn. (Damage and effects that say "destroy" don't destroy it.)