'a problem as a challenge' vs. 'a problem it a challenge'

[Foka]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #85 "Verbal Skills Test", question 10

Let's look at it this way -- it's not so much a problem ......... a challenge to our efforts.

(a) as
(b) since
(c) for
(d) it

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #85 "Verbal Skills Test", answer 10

Let's look at it this way -- it's not so much a problem as a challenge to our efforts.

Correct answer: (a) as

Your answer was: incorrect
Let's look at it this way -- it's not so much a problem it a challenge to our efforts.
_________________________

please explain why the answer is "as".

Foka

[Mister Micawber]

.
So...as is a pair of correlative coordinators, and they need each other in a sentence:

It is not so much my foot as it is my ankle that hurts.
I am not so hungry as you are.
There are not so many guests here now as an hour ago.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber