Distinguishing between adjective and adverb for word inserted between article and noun for Lückentext

Question

Da der reibungslose Transport von Waren und Versorgungsgütern in Deutschland nach wie vor auf einwandfr(?) funktionierende Straßen angewiesen ist, müssen Bundesstrassen, Autobahnen und Brücken stets repariert und für den Schwerverkehr befahrbar sein.

For my university entrance exam, I have to write an exam where I have to fill some deleted text. In the above question, for example, I would have to figure out what goes in place of "?". Here the answer is "ei" because einwandfrei is an adverb as discussed in this post.

When I wrote the answer, I wrote "eie" thinking it is an adjective in the nominative feminine case/same ending as "funktionierende". Another reason is that, streets can indeed be "einwandfreie"... but... this is wrong.

How could I have figured out methodically that it must indeed be einwandfrei?

Answer 1

It's also acceptable with very similar meaning to write

... einwandfreie, funktionierende Straßen...

But mind the comma in the sentence: the two adjectives are a list which needs separation by a comma or for a list of two more commonly a conjunction like 'und' or 'oder'.

Yet it would have been arguably difficult to say that with a comma the sentence would be wrong. Yet the more common way to formulate is how your teachers want it. It's basically the same in English: what is more common to say?

they require a proper, functioning road network...

or

they require a properly functioning road network...

Arguably a road network does not need to be proper or immaculate to function properly. So the latter phrasing with an adverb to describe and qualify the way of functioning rather than a list of equal properties to describe the road network itself is the more logical completion - as expected in your exam.