Elemental Resonance

• WWizards of the Coast2006-05-01
  Official Ruling
  This ability isn't a mana ability. It uses the stack.
• WWizards of the Coast2006-05-01
  Official Ruling
  Only the first main phase in a turn is the "precombat main phase," even if additional main phases are generated by some effect.
• WWizards of the Coast2006-05-01
  Official Ruling
  If the enchanted permanent's mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller adds . If its mana cost is , this Aura's controller chooses whether to add , , or . The choice may be different each time the ability resolves.
• WWizards of the Coast2009-10-01
  Official Ruling
  You get the mana whether you want it or not. If you don't spend it, it will disappear at the end of the current step (or phase).

Rulings via Scryfall