The correct answer is A.
The strongest indicator that your friend is suffering with depression is that she rejects all social invitations, especially since she used to be very social. People with depression can isolate themselves from others and will not engage at all socially. They cannot deal with other people or the emotions that they are feeling so they close themselves off from others. If you friend is angry with you or has gone out of town without telling you then these are likely indicators that you she feels you have done something wrong or maybe feels like she can't trust you these days or family reasons that she can't tell you about.