In this context abgeschlossen is the only correct word given the choices: You are writing a job application (or something like that) and you have completed your Masters successfully (I assume). Abgeschlossen is just the right word for that situation.
Other words: vollendet, absolviert.
Beenden in this context is weird, because it does not imply that you have finished it successfully. It could mean that you dropped out:
Ich habe mein Studium beendet, weil es mir keinen Spaß gemacht hat. (quit)
Obwohl es mir keinen Spaß gemacht hat, habe ich mein Studium beendet. (finished)
Aufhören is more like spoken language and I think it would be inappropriate in a job application. Like beenden it does not say wether you finished successfully. It is inclined a little bit more in the direction of actively stopping something:
Ich habe aufgehört Mathematik zu studieren und habe zu Germanistik gewechselt.