The sentence that does not contain any punctuation errors is A) Thinking about it now, I realize that I was wrong. There has to be a comma after <em>now, </em>because the whole order in the sentence is inverted.
The answer to the English question presented above would be letter a. Thinking about it now, I realize that I was wrong. The sentence that does not contain any punctuation errors would be 'Thinking about it now, I realize that I was wrong.'. All the other choices are not applicable.