Answer:
The volcano that erupts frequently accumulates less lava (because it is being erupted frequently) so the eruptions are less massive and more controllable, but it's actually dangerous.
The other one, the inactive volcano is "safer" most of the time, the fact that it was active 100 years ago means that there is a possibility that it erupts in a given time, but with only this information you can't contemplate the actual danger of the eruption, it may be kinda small or cause a natural disaster.
So both of them are dangerous, the one that erupts frequently is most consistently dangerous, and the other has a probability of being really dangerous, but most of the time is not, then the right answer is:
"a volcano that erupts frequently"
