[susan53]

B is OK grammatically and means the same as A, but English doesn't like negating an opinion - usually the "opinion verb" is negated : Eg the opposite of I believe/think that Japan was wrong is I don't believe/think that Japan was wrong
or I believe think that Japan was wrong. With believe it's just possible to negate the opinion : I believe that Japan wasn't right though I suspect it would be less common than the others, but with think it's not possible: * I think that Japan wasn't right ( The asterisk indicates an incorrect sentence.)

The real problem with your sentences is the presence of the word strongly. C changes the meaning. Your sentence is : I don't strongly believe that Japan was right here, not no longer negates believe but strongly . So the meaning becomes : I believe that Japan was right but not strongly.

So, in answer to your question, the answer to "What's the opposite of I strongly believe that Japan was wrong..."
is most likely to be I strongly believe that Japan was right.... But if you want an equivalent, B is possible but less likely than A, and C changes the meaning completely