What or How? (How/What he did it is a mystery.)

[Huda2]

hello,
in the following , I'm confused between b and c
which best?
_______ is a mystery. I wish that I could ask him.

A. How did it
B. What he did it
C. How he did it
D. When did it

[Kitosdad]

C.

A. How did it ... the method he used........ who ? = He .

B. What he did it ...... grammatically incorrect.

C. How he did it, is a mystery to me.---------------- correct.

D. When did it......... the time " it " was done.
_________________
Keep it simple ... Keep it interesting.

[Huda2]

are there any rule for choosing c?

[Kitosdad]