If east and west germany had not reunified, it is most likely that
a)east germany and west germany would have remained equally strong.
b)west germany would have grown much stronger than east germany.
c)east germany and west germany would have grown increasingly weak.
d)east germany would have grown much stronger than west germany.

The right answer is b).

West Germany was a lot stronger at the time of reunification and the difference can be still seen today, with eastern states a lot poorer than the western ones.
One of the reasons was that the socialist system of the East was not as productive as capitalism in the West. There was little competition, and the economy was decided on a political level, rather than on the need and real potential. Additionally, export was limited to other Eastern Block countries, who were not much better off themselves.
Among many, many reasons for the failure of the socialist system was that it was imposed on the East (by the Soviet Union), rather than chosen by the people, and in order to keep the system from changing, a massive surveillance of the citizens was in place, with the secret police being a real threat to many people. For most people it was more important not to "stick out", and not to be accused of treason, and there was little incentive for the individuals to contribute to the economy.