Hello, Yale Wale,
I'd choose (C) (notice that (A) and (D) are identical). The genitive case ('s) is rarely used in these cases.
In general, nouns in attributive position will only be used in the plural when, rather than refer to a general class, they involve a variety of different individual elements. In this old thread you can find some interesting reflections and examples. Notice that, even if the rule used to say that attributive nouns should take the singular form, Quirk says that "the plural attributive construction is on the increase..." (Quirk et al., A Comprehensive Grammar of the English Language, Longman, 1985, pp. 1333–1334).