When galaxies collide, the event that can happen is:
Galaxies merge.
The other options provided, such as galaxies disappearing, destroying their stars, or tripling in size, do not accurately describe the typical outcomes of a galaxy collision. While some stars within the galaxies may be affected, galaxies themselves do not simply destroy their stars or disappear entirely as a result of a collision. Instead, they often interact gravitationally and can eventually merge into a larger galaxy.