Question

"Indeed they did. But history can be careless about who it remembers" Is this a simile or an idiom or a personification? Why?

Answer 1

Idiom.
It is not a simile because it is not comparing that item or thing to another item/thing.
Ex: As brave as a lion. as crazy as a money.
Personification is telling someone's emotion and feelings so it is not this one.

Answer 2

It's personification because history is not a person yet it is exemplifying human traits like being careless and remembering.