I think "however" is the right answer no matter what the key says -- especially if this sentence is in isolation.
Context (especially the sentences that immediately surround this one) might make a big difference; but in isolation, the sentence with "whereas" filling the blank doesn't make much sense.
Is this a selection from a longer passage? Is the gist of the thing that people are unsatisfied BECAUSE they are getting two marks? Then the blank should be filled with "therefore."
Or is the idea that people are dissatisfied IN SPITE of getting two marks?
I can't readily come up with a backstory to fit "whereas' into this sentence.
Student or Learner