Historical Rules · 3 May 2019
Rule 702.43a
Official source text from the 3 May 2019 Rules release
Sunburst is a static ability that functions as an object is entering the battlefield. “Sunburst” means “If this object is entering the battlefield as a creature, ignoring any type-changing effects that would affect it, it enters the battlefield with a +1/+1 counter on it for each color of mana spent to cast it. Otherwise, it enters the battlefield with a charge counter on it for each color of mana spent to cast it.”