Answer: The right answer is the B) If she was a more active person, she would feel better.
Explanation: Just to elaborate a little on the answer, it can be added that, with the exception of option C, these are examples of conditional sentences in English. There are various types of conditional sentences (zero, types 1, 2 and 3, and a mixed type) and most of them contain the word <em>if</em>. They are used to speculate about what could or might have happened, or about what we wish would happen.
Although it is not incorrect to say "if she was" in certain contexts, specifically when referring to probable or possible situations, it is incorrect to use it when referring to sutuations that are not true, or that you wish were true. Since the subject in this sentence IS NOT an active person, then "were" should be used instead.
