Question

drag the missing word into place
and though we can’t speak for those whose lives were lost,
we must ask ourselves, how did the happen?
how can we make sure it won’t happen again?
genocide
aryan
ghetto

Explanation:

The sentence is asking about a tragedy involving lost lives and preventing its recurrence. "Genocide" refers to the deliberate killing of a large group, often an ethnic group, which fits the context of a catastrophic event. "Aryan" is a racial/ethnic term, and "ghetto" is a segregated area, neither fitting the "happen" context as well as genocide.

Answer:

genocide