Final answer:
If Avani buys all the raffle tickets, her probability of winning the grand-prize mountain bike is certain or 100%.
Step-by-step explanation:
The probability that Avani will win the raffle if she buys all the tickets would be certain or 100%. This is because she owns all possible outcomes of the raffle, making it impossible for her not to win the grand prize. In terms of probability, when Avani has all 100 tickets, her chances of winning are 100 out of 100, or 1, which translates to a guaranteed win.