The correct option is A a
Let’s examine the sentence: “If I was the king, I would change the face of my country.” The sentence talks about an imaginary situation and so it would use the verb ‘were’ in the ‘if’ clause. "I were" is called the subjunctive mood, and is used when you're are talking about something that isn't true or when you wish something was true. A verb is in the subjunctive mood when it expresses a condition which is doubtful or not factual. It is most often found in a clause beginning with the word if. Hence, the error is in Part A of the sentence. So, Option A is the correct answer.