Hi NoAgenda, I feel that the correct answer here is D "one", as it is used in pair with the word "two". Why not ”single"? First of all it would require an article, and it doesn't match as well with "two" as it does with "double“. Why not "a"? Because when you read this sentence you probably read "s" as [es] therefore "a" doesn't fit at all. Why not "an"? Even though the basic meaning of an undefined article is "one" here it would not fit well because it shifts the meaning from the numbers of 's'-es to the nature of the questioned letter. For example: Do you spell your surname with an 's' or with a 'z'? Hope this helps.

To my knowledge, the usage of "a" and "an" before a word is determined by the vowel sound and not the spelling. This is why we use "an' before "honest" since the pronunciation of the first syllable of "honest" is a vowel sound. Similarly, FOR INDIVIDUAL LETTERS, we use "an" before A, E, F, H, I, L, M, N, O, R, S, X. For example, is there an MTR station nearby? WOULD SOMEBODY POINT OUT MY MISTAKES. For the answer to the question, B is definitely out. It seems that C and D are ok, but I prefer D. Hope some experts will clarify more.