You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.
Each Dragon you attack with will cause Dromokaâ€™s triggered ability to trigger. For each instance of the ability, youâ€™ll determine which creature to put +1/+1 counters on as it resolves. Specifically, the creature you control with the lowest toughness as the first such ability resolves may not have the lowest toughness as the second such ability resolves.
Bolster itself doesnâ€™t target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptainâ€™s bolster ability.
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