Feat of Resistance is a common Instant card from Khans of Tarkir (KTK), collector number 10. Current market price: $0.19 (foil $0.38), TCGplayer market prices via Scryfall.
Put a +1/+1 counter on target creature you control. It gains protection from the color of your choice until end of turn. (It can't be blocked, targeted, dealt damage, enchanted, or equipped by anything of that color.)