The correct option is B As long as
Option B: 'As long as' refers to a condition. It means 'provided that' or 'on condition that'. It relates to the given sentence, which means: 'Provided that we are working, we are happy and healthy.'
Hence option B is correct.
Option A: 'As far as' refers to distance. It means 'for as great a distance as'. The given sentence does not refer to distance, hence option B is incorrect.
Option C: 'Instead' refers to an alternate option, 'in the place of'. When placed in the sentence, it does not sound complete. Hence C is incorrect.
Option D: 'Nevertheless' means 'in spite of that'. This word, too, does not complete the sentence in meaning. The sentence requires another clause, or another sentence to complete its meaning. Hence option D is incorrect.