The pronunciation by DUO is wrong! Arkene is in Danish pronounced 'arkene' with a stress on the first part of the word and not 'arkeene where the stress is on the second part of the word.
Agree. The trouble is due to DUO linking up to Google translate, which is wrong too! Someone at DUO should try to just listen to the course before it is launched. Or tell Google Translate that they should improve their pronunciation.
In this sentence, "Fuldstændigt" is an adverb modifying the adjective "hvide." Typically, adjectives agree with the nouns they modify, but adverbs are invariable. In this example, the ending -t on "fuldstændigt" helps you to recognize the word as an adverb. Does that help?
Ethymologically it looks much closer to "completely" if I assume the similarity with German... fuld-stændigt sounds the same as vollständig (which is also broken into pieces voll-ständig, and also means completely).