A.32.1- The ability is written "Provoke".
A.32.2- A creature with "Provoke" means "Whenever this creature attacks, you may have choose to have target creature defending player controls block this creature this combat if able. If you do, untap that creature." [Legions FAQ 2003/01/23]
A.32.3- Provoke is a cumulative ability. Having Provoke more than once will cause each Provoke ability to trigger. For example, a creature with Provoke twice will get to choose two defending creatures. [Legions FAQ 2003/01/23]
A.32.Ruling.1- You can choose a creature that is already untapped if you want to. [Legions FAQ 2003/01/23]
A.32.Ruling.2- If the chosen creature is unable to block for any reason (such as becoming tapped before blockers are assigned) there is no penalty. But if it can block this attacker, then it must. [Legions FAQ 2003/01/23]
Note- Also see Must Attack or Block, Rule C.7.