Dromoka, the Eternal — ruling 1

Each Dragon you attack with will cause Dromoka’s triggered ability to trigger. For each instance of the ability, you’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.

Dromoka, the Eternal — ruling 2

Bolster itself doesn’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 Skycaptain’s bolster ability.

Dromoka, the Eternal — ruling 3

You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.

Dromoka, the Eternal : règles, fonctionnement et interactions (MTG)

Dromoka, the Eternal is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever a Dragon you control attacks, bolster 2. (Choose a creature with the least toughness among creatures you control and put two +1/+1 counters on it.)

How it works : Dromoka, the Eternal

Original English text — Wizards of the Coast / Scryfall

#1November 24, 2014· Wizards of the Coast
Each Dragon you attack with will cause Dromoka’s triggered ability to trigger. For each instance of the ability, you’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.
No other card is named in this ruling.
#2November 24, 2014· Wizards of the Coast
Bolster itself doesn’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 Skycaptain’s bolster ability.
No other card is named in this ruling.
#3November 24, 2014· Wizards of the Coast
You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.
No other card is named in this ruling.