Final answer:
The correct spelling for the word formed by adding '-ly' to 'due' is 'duly' (option C).
Step-by-step explanation:
The proper spelling of the word formed by combining due and -ly is C. duly. When forming adverbs in English that end in -ly, you typically add the suffix to the base adjective without changing its spelling, unless the base word ends in y, in which case you replace the y with i before adding -ly. The correct spelling does not include changing the base word or adding extra letters that were not there originally.