According to the kwiz, there are 2 correct answers to the question:
haya ganado ( my answer)
ganara ( imperfect subjunctive)
The page on the subjunctive does not address using these subjunctives interchangeably. I learned that when the subjunctive trigger is in the present tense ( no está seguro) and the event being considered is in the past ( won the competition), the present perfect subjunctive is called for. Do you have a lesson the the distinction?