B is incorrect because it switches up the sufficient and necessary language in the conclusion of the argument. The argument states that in order for computer to translate a user's spoken words into written text, it's voice recognition technology needs to be improved.
What answer choice B does is flip this and confuses the sufficient and necessary. This doesn't provide the necessary assumption for this argument.
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further!