The part of speech of ‘due to’ remains a bone of contention for some grammarians, conservatives considering it an adjective only and liberals accepting its long use as an adverb as well. Here is a bit of the history of this discussion:
Main Entry: due to
: as a result of : BECAUSE OF
usage-- The objection to ‘due to’ as a preposition is only a continuation of disagreements that began in the 18th century over the proper uses of ‘owing’ and ‘due’. ‘Due to’ is as grammatically sound as ‘owing to’, which is frequently recommended in its place. It has been and is used by reputable writers and has been recognized as standard for decades. There is no solid reason to avoid ‘due to’.
OED says “be due to” has been used in the sense of “be ascribed to” or “be caused by” since the middle of the 17th century. According to this Dr Samuel Johnson was somehow critical of this adjectival usage saying “the use may be proper but unusual”. The adverbial use of “due to” in the sense of “because of” first appeared in literature in 1897. However William A. Craigie first criticized this as “erroneous” and a more harsh critique came from Henry W. Fowler, who in his “Modern Usage of English” (1926) censured as “illiterate” the use of “due to” to mean “because of”. The adverbial use, however, is now widely current though rejected by many grammarians.
The author of your quiz may be on the side of the brothers Fowler.