Guivre dévastatrice

Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities are activated abilities and will have colons in their reminder text. An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate.

If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.

If the target of Ravager Wurm's first mode isn't a legal target as that ability resolves, or if Ravager Wurm has left the battlefield, neither creature will deal or be dealt damage.

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.

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.

You may choose neither mode for Ravager Wurm's triggered ability. In this case, the ability is removed from the stack with no effect.