the correct answer is definitely (c).
only (c) and (d) contain a verb that is properly parallel to "have produced", and choice (d) contains the unidiomatic "possible for ___ing" while (c) contains the correct version, "possible to (infinitive)".
--
this is a special idiomatic construction in which the "it" actually doesn't have to stand for anything.
i posted about that construction on
this thread; go have a look.
yes, i know, exceptions to rules are not fun. but "it" is a rogue pronoun this way; there are just a handful of constructions in which it doesn't have to stand for anything.