I wrote "Why is skin red?" and was marked as incorrect. The correct answer given was "Why is my skin red?", yet the translation/meaning given here is "Why is the skin red?" Was my answer correct or incorrect and why?
Your translation would be general, rather than about some specific person's skin. Therefore, the postposition would be 는. Without context, possible translations could employ the, my, your, our, their, his, hers, its.