Riot is a replacement effect. Players can’t respond to your choice of +1/+1 counter or haste, and they can’t take actions while the creature is on the battlefield without one or the other.
25-01-2019
wotc
If you choose for the creature to gain haste, it gains haste indefinitely. It won’t lose it as the turn ends or as another player gains control of it.
25-01-2019
wotc
If a creature entering the battlefield has riot but can’t have a +1/+1 counter put onto it, it gains haste.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.