Crystalline Giant
At the beginning of combat on your turn, choose a kind of counter at random that this creature doesn't have on it from among flying, first strike, deathtouch, hexproof, lifelink, menace, reach, trample, vigilance, and +1/+1. Put a counter of that kind on this creature.
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Official rulings (3) · Scryfall
As long as Crystalline Giant doesn't have all ten of the listed counters on it, you'll put one on it at the beginning of combat on your turn. If it has all ten kinds, the ability will trigger but you won't put any counter on it.
If Crystalline Giant has one of the listed keywords but not because of a counter of that kind, that kind of counter is still eligible to be chosen at random.
You choose a kind of counter at random and put it on Crystalline Giant all while its ability is resolving. Players can't take actions between the time the choice is made and the time Crystalline Giant has the counter.