Final answer:
The word 'formally' is used incorrectly in the sentence. The correct word should be 'formerly', which refers to something that existed in the past.
Step-by-step explanation:
In the provided sentence, 'The professor was formally an intern in this very hospital back in the 1980s', the word used incorrectly is (b) 'formally'. The correct word should be 'formerly', meaning 'in the past'. 'Formally' refers to something done in accordance with rules or etiquette, which does not fit the context here. So, the correct sentence should be: 'The professor was formerly an intern in this very hospital back in the 1980s'.
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