Toco el piano _________ hace dos meses.
Translation:
He/She has played the piano [blank] two months ago.
The options are as follows.
desde - "since"
se - "-self" [reflexive pronoun]
les - "them" [indirect object pronoun]
por - "for"
Neither a reflexive pronoun nor an indirect object pronoun work here, so those two options can be eliminated immediately. Of the two remaining options, desde clearly makes more sense than por - and it is correct.
Answer:
desde
Toco el piano desde hace dos meses.
