Right answer: gastamos
This is a sentence written in simple past tense, we are able to know this because of the world ayer (yesterday) at the beginning of the sentence.
In addition, the Spanish verb gastar (to spend) is conjugated with the 1st person in plural nosotros (we), which are Ana y yo (Ana and I), as gastamos (we spent).
Therefore the complete sentence is:
Ayer, Ana y yo gastamos 20.000 pesos para comprar ropa
Yesterday, Ana and I spent 20,000 pesos to buy clothes
The other options are incorrect because do not match with the subject of the sentence:
-gastaron (They spent) although is in past is conjugated with the 3rd person in plural.
-gastaste (You spent) although is in past is conjugated with the 2nd person in singular.
-gastó (He/She spent) although is in past is conjugated with the 3rd person in singular.
