Question

Which of the following tells what you have just done?

Estoy bailando.

Hace tres días que bailo.

Hace tres días que bailé.

Acabo de bailar.

Answer 1

Let's translate these...

x Estoy bailando. => "I am dancing." That's something currently in progress.
x Hace tres días que bailo. => "It's been three days since I've been dancing." That'll be in progress.
x Hace tres días que bailé. => "It's been three days since I danced." That's a past action.
!!! Acabo de bailar. => "I just danced." a

"Acabo de bailar" is the answer, since the verb "acabar de+infinitive" means "to just finish+an action."