I would normally write the (b) version, but I think that the (a) version is possible is some contexts. The (a) version is more likely in speech than writing; word stress is fairly important.
I am sure that the(a) version would not be penalised in any of the main Britsh examinations. However, learners should be aware that the (b) version is nearly always the natural choice for native speakers.
Student or Learner