Historical Rules · 29 September 2017
Rule 702.43a
Official source text from the 29 September 2017 Rules release
Sunburst is a static ability that functions as an object is entering the battlefield from the stack. “Sunburst” means “If this object is entering the battlefield from the stack as a creature, it enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. If this object is entering the battlefield from the stack and isn’t entering the battlefield as a creature, it enters the battlefield with a charge counter on it for each color of mana spent to cast it.”