'of all the reasons' vs. 'of all reasons'

'of all the reasons' vs. 'of all reasons'

Given:
Of all ____ reasons for my decision to become a university professor, my
father's advice was ____ most important one
Which of the following choice is correct?
A: the; a
B: /; a
C: /; the
D: the; the
The slash ("/") here means "don't fill anything".
The author says the correct answer is D, but I think C is also correct.
Why is "the" required before "reasons"? What is the explanation from a
grammatical point of view?
Actually I am an EFL teacher and my native language is Chinese. I am quite
confused about the question. I've asked the question somewhere else, but
they failed to explain why "of all the reasons" is grammatically correct
while "of all reasons" is not. Or do you also think they both are correct?