"A la población en general."
Translation:To the population in general.
A means to. There are some other possibilities, but neither in or at would generally be one of them. The preposition a generally implies motion, real or abstract, toward something. Both in and at as well as on are generally covered by en. Él está en la casa. He is in the house. Puse el asado en el horno. I put the roast in the oven. Él está en el trabajo. He is at work. Alguien está en la puerta. Someone is at the door.
I am assuming that you are talking about the hover drop down menu. Those are hints,, not answers. I can't think of an instance where you would translate a la as over, but there may well be. Prepositions can play many roles in languages. But a la is really a basic phrase that you use all the time in its normal translation as to the (before a feminine object)
Of course you will have fragments, especially pieces like this which could be part of many sentences. There was a user decrying that another sentence was too long, and that was only seven words. This fragment is five. Beginners need to take things slow.
It actually isn't a complete sentence. But Duo does use interesting parts of sentences for exercises. The sentences that this phrase might have come from would probably be longer and more complex than some students would like. But if you learn how the parts come together, a complex sentence becomes easier.