Prep SC Q:

[RonPurewal]

1, the usage of not so much as, does as always need to be there?

^^ "not so much" CAN appear by itself, if the other part of the comparison is just IMPLIED (i.e., not actually written at all).
e.g.,
The property owners replaced the old statue with one that was not so much a reminder of the building's unpleasant past.

if both parts of the comparison are actually written, though, it's "not so much X as Y".

[RonPurewal]

2, I automatically assumed the usage not...but in choice C, however there is "so much" after not, does it matter putting so much between not...but?

that construction isn't being used.
in the construction "not so much X as Y", "not" isn't negating "X"—it's negating the "so much" part. these are 2 completely different constructions, with 2 different meanings.

3, what is the differences between "not A so much as B" and "not so much A but B" if the latter one is idiomatic

the latter one isn't a thing.