I got this answer wrong but from my understanding these are the following reasons why answer choice (B) is correct:
While the relative costs of concrete and steel are a predominant consideration in the design of a skyscraper,
of no less importance to civil engineers is the time needed to fabricate the structural elements and the distance of the project site from the production sites of the raw materials.
of no less importance to civil engineers is the time needed to fabricate the structural elements and
the distance of the project site from the production sites of the raw materials
Here there are 2 subjects- Time and distance.
Considering this we need to use the plural verb 'are'.
X and Y are going shopping.
While the relative costs of concrete and steel are a predominant consideration in the design of a skyscraper,
Time and distance are of no less importance to civil engineers.
A. of no less importance to civil engineers is the time needed to fabricate the structural elements - Eliminate as wrong verb 'is'
B. of no less importance to civil engineers are the time needed to fabricate the structural elements' - keep looks good.
C. of equal civil-engineering importance are the time in which the structural elements are fabricated- I've read through the other answers, but the first thing that came to mind was that civil engineering is kind of an adjective here and the noun is importance but importance to who? so it didn't really make sense to keep this as an option. (If anyone sees this, please let me know if this is even correct, thanks!) - Eliminate
D. for a civil engineer, of no less importance are how long it takes to fabricate the structural elements- does not seem correct because it would make more sense to use the words 'to' in this sentence to indicate purpose than for. The goal is to design a skyscraper. (Again, it could be incorrect) Further, importance is singular noun so it should be followed by the verb 'is' and not 'and' -eliminate
E. of equal importance to a civil engineer is the amount of time needed for fabricating the structural elements - we don't need to use amount of time here. we can use the noun in its standard form as distance is in its standard noun form. - eliminate
Learnt quite a bit with this question!