... get better ...

The 2nd one imo
better wait for an English expert though

Combining human and technological resources to the best advantage is a complicated business. States become better at these calculations through practice.

States = Countires?


I forgot to ask ...
States = Countries?
States = USA?

Could be either. Without context I suspect the emerging African nations, but that is only a suspicion, not a proven fact.

grammar tells me ...
"States get better at..."
I noticed agreement of plural subject and verb, so it should mean "countries..."
Am I right?

countries, nations, states...

It could mean states as in the constituent parts of the United States, or it could mean separate nations. It cannot mean "States get better as a result of . . ." because the quoted sentence does not explain why states "get better at" anything. It would mean, however, that states improve their ability to do something, whether or not it explains why this is so.