The correct option is A Had I been a little early, I would not have missed the train.
All the statements have the correct combination of tenses. So, we need not check it. Let us find the errors in other statements. 'Early than now' and 'earlier' suggest that I reached early; in that case, I did not miss the train. So, choices (b), (c), and (d) do not state the same idea as in the question statement. Hence correct choice is (a).