When galaxies collide, the most likely outcome from the options given is that galaxies merge.
While galaxies may interact in ways that lead to star formation and the redistribution of stars, they do not typically destroy their stars or simply disappear. The idea that galaxies could triple in size isn't accurate as well; they can grow through merging, but the size change is more complex than a simple multiplication.
So, the correct response is: Galaxies merge.
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