Hello Hirashin,
Yes, both of those work. They are both identical to b).
There could be a slight nuance, such as;
c.) Could mean that you had sufficient skill, but that during that particular time you became nervous or confused or something, and it caused you to speak clumsily. Or perhaps you spoke broken Japanese on purpose, but now you regret it and wished you had spoken without such affectation.
As I said, it is just a slight, possible nuance, and people would only catch this nuance from the context. In general, all three (b, c, and d) work when speaking about an experience in the past.