Each of these words mean different, and would totally change the meaning of the sentence.
The correct answer is "Yet" because it's the only word that would provide the exact meaning.
Here, the sentence means that some new service upgrade was introduced, to lets say some bank accounts, and was expected to be successful but few people used it (so it did not come out as successful as the producers thought it would). So, yet would fit here.
Although would change the meaning in the context that few people upgraded their accounts, still the producers expected the service to be a success.
OR Despite the fact that few people upgraded their accounts, the producers expected the service to be a success