Yes, the wording of statement 2 is ambiguous. Or maybe we are just missing some subtlety of English

1: Two adjacent faces of the solid have areas 15 and 24, respectively
NOT SUFFICIENT - for sure

2: Each of the two opposite faces of the solid has area 40.

There are 2 ways to interpret this:

one:-
Each of the two opposite faces ==read as== each pair of opposite faces
In that case it is a cube with side 2root10
So, B is SUFFICIENT and so answer is B

two:-
Each of the two opposite faces ==read as== one pair of opposite faces
In that case their explanation works and so answer is C

Our question is what is the English meaning of "Each of the two opposite faces" mean, rt?

Does it mean "each of - two opposite faces" or
does it mean "each of two - opposite faces"

This reminds me of the famous punctuation ambiguity in sentence ""A woman without her man is nothing."
A man's way: "A woman without her man is nothing."
And woman's way: "A woman: without her, man is nothing."

Similarly in OG12: Q61 on Page 278, explanation on page 308.

Is double scoop = 2 single scoops? or a double scoop is a scoop of its kind and a single scoop is another kind?