The correct answer is D. Whether
Explanation:
In writing, the word spelling refers to the way a word should be written which includes the letters of a word and the order of these. This factor along with the context provided in a sentence determines the word that is correct to complete a sentence. In the case of the sentence "I could not decide __________ I would dress up as a superhero or as a ghoulish monster" the missing word introduces the election between two different options and from the options provided the only word that can be used for doing this is "whether" as this word expresses two or more different options. Also, this word is the one that is correctly spelled, as other options have different letters and also different meanings that do not fit with the context, "weather" refers to the climate and "wether" refers to a male ship.
