« Un billet aller » is not being listed as a correct option for 'one-way ticket' but should be. There are several choices given for using « aller simple », which implies this is the only correct term.
Freeform Writing Exercise A1
When buying a train ticket, we ask for 'un aller' or 'un aller simple'. You might stretch to 'l'aller - un aller' (as in 'j'achète juste un aller / l'aller') in a conversation.
I hope this is helpful.
Bonne journée !
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