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The government issued a warning to local companies that all waste must be disposed of ................. of they will face heavy fines.
a) corrects
b) correcting
c) correction
d) correctly

Which one is correct? and why?
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AnonymousWhich one is correct?
First, what do you think? (Hint: the missing word modifies the phrasal verb disposed of.)

The government issued a warning to local companies that all waste must be disposed of _____ or they will face heavy fines.
Comments  
Assuming that you meant: "The government issued a warning to local companies that all waste must be disposed of ................. or they will face heavy fines", the right answer is "correctly". It's a adverb of mode, indicating the way how you dispose the waste of. But, just to make the sentence slightly better (and maybe also to help you understand it better), I would also add a comma before the "or".
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 RandomGuy's reply was promoted to an answer.
Correction