The correct answer is 'Owing to.' Key Points - With regard to is a preposition that signals a reference to the subject of a sentence. - Example: With regard to dinner, my vote will always support pizza - Due means expected or planned to happen or arrive. - When combined with the rest of the sentence, it functions as an adjectival prepositional phrase. - Example: The rent is due on the fifteenth of each month. We can use as to introduce two events happening at the same time. - Example: As I knew you would be late, so I left early. - Owing to indicates that something happened as a result of something or introduce the reason for something happening. - Example: I missed my flight owing to a traffic hold-up. - Correct sentence: Owing to his ill health, he retired from his business.