Answer:
The correct answer is: a) What someone has just done.
Step-by-step explanation:
Acabar de + infitive is used to express an action that has just happened.
Acabo de llegar a casa. (I have just come home.)
Acaban de llamar a mamá. (They have just called mum.)
To form this expression, we use the verb acabar in the present tense:
1. acabo
2. acabas
3. acaba
1. acabamos + DE + infinitive
2. acabáis
3. acaban
It refers to the actions that have just occurred (according to the moment of speech).