score:5
La Molina's own website says that there are train and bus options.
The bus is provided by SagalΓ©s and runs once daily, departing Barcelona at 06:30 and (if I understand correctly) departing La Molina at 16:00. I'm not sure how much time that would give you on the slopes. Note that it is dependent on there being enough interest on a given day.
The train is Renfe Rodalies (middle-distance), and would probably allow you more time on the slopes because the first departure from Barcelona is at 06:08 and the last departure from La Molina is at 19:24. What isn't clear is how to purchase the package which includes the train ticket and the ski pass; perhaps it's best done in person on the previous day.