I would actually think evolution a particularly poor choice.
If you want to pick one question to ask (and if we leave aside the obvious criterion of easy detectability from space) then you would want to pick one strongly connected in the dependency graph. Heavier than air flight, digital computers, nuclear energy, the expansion of the universe, the genetic code, are all good candidates. You can't discover those without discovering a lot of other things first.
But Aristotle could in principle have figured out evolution. The prior probability of doing so at that early stage may be small, but I'll still bet evolution has a much larger variance in its discovery time than a lot of other things.
digital computers
This is a good one. I like it.
nuclear energy
Seems dependent on substitute energy availability and military technology.
the expansion of the universe
There seems to be significant variance in how much humans care about such things, and achievement depends significantly on interest. Would aliens care at all about this?
If you want to pick one question to ask
I think we would do quite poorly with any one such question and exponentially better if permitted a handful.
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