Reach
When Sandsteppe Mastodon enters, bolster 5. (Choose a creature with the least toughness among creatures you control and put five +1/+1 counters on it.)
You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.
24-11-2014
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.