Translation:What is most important is that we do what we can.
"The most important" can't moonlight as a noun and stand as a subject on its own like "det viktigste" can in Norwegian. In English, there's always a noun or pronoun in the picture.
You can solve this either by translating it as "the most important thing", by expressing it with a pronoun, "what is (the) most important", or by rephrasing it completely: "it is most important that...".
So basically because english is weird. Kinda confusing that duolingo gives 'The most important' as an possible translation when you mouse over 'Det viktigste'.
Yes, or Norwegian, depending on how you see it.
Hints are shared between sentences, so that hint would be correct for a sentence like "Filosofi er det viktigste faget" ("Philosophy is the most important subject.").
Usually, the top hint for a given word or phrase will be the correct one, but there are cases where that doesn't happen as the hint algorithm has a couple of flaws. For example, it doesn't know what to do when the same word appears twice in a sentence, but with different translations, something that's not uncommon for prepositions to do. Multi-part hints are also prone to interfering with each other.
I wrote, "that which is most important is that we do what we can". Could that be acceptable
Yes. Provided you have access to it, please use the report function to report alternative translations.
"Det viktigste er at vi gjør det vi kan."
Why use Det instead of Hva ?